Did your vet discuss with you the location of the lumps, and have you discussed this with them now? If his hind legs are not functioning, it seems that the surgery may have been on or around the spinal cord, and whether it is bruising that may be causing a swelling around the cord, cutting off temporarily the ability to use his hind legs or whether something went wrong with surgery, it is the duty of your vet to have told you the risks, the possibility of this happening during the recuperative process, and if it is something that is permanent, that is something they needed to discuss with you also. I hope that it is post-operative swelling and that it will go down, returning to him the full use of his back legs. The best advice is to follow your instincts after you speak with the vet and have them examine your dog. You know your dog, you also have good instincts, and unfortunately, we're always told not to trust our hunches. If something doesn't sound "right" to you, it's your job to follow that hunch and be an advocate not just for yourself, but for your dog, and you should consider a second opinion. Your vet should not get angry at this, it happens all the time, and if they do, that is a sign that they either have some kind of God complex or they have something to hide. In any event, I would take him back for an exam, and make sure that before you walk out that door, you have asked every question (don't be intimidated by a doctor or a vet or a lawyer, you are paying for their time, and do not let them rush you out until, with respect to the issue at hand, you know almost as much as the vet does, whatever amount of information that gives you the ability to understand everything about it.)
Based on what you didn't put in the question and the question itself, it doesn't seem like you got any explanation, and that was a failure on the part of the vet. I don't buy into any professional, especially a vet, who in reality has two clients, you and your pet, to provide competent and professional care and information involving your pet's health, trying to sell you any Brooklyn Bridge about it being your duty to ask questions and you didn't, so they had satisfied any standard. How can you ask questions if you don't know the facts. If you are timid, prepare a list ahead of time of questions that you have, and put it in a notebook with each question on a separate page, so that when they answer it, any questions arising from that can be written on the same page, asked at the same time, and addressed. Again, a vet that doesn't have time to do this with you or who makes you feel like you are taking time away from their precious day and who it seems (and I don't know from the brevity of the question) was perfectly content for some reason not explaining the nature of the surgery (which again it seems that they were close to or maybe on the spinal cord), the potential recovery issues, so that you could prepare for them and know, and sending you home with this fear and uncertainty as to what is going on.
To sum it up, I would do the following:
1) Make an appointment with your vet to have your dog examined, and write the list of questions (In situations like this, I always take a notebook with questions, because I want to make sure I don't forget to ask something, which I usually do if I don't have my trusty list, and my vet is always, always patient and takes as much time with me and my animals as is needed so that my pet is well taken care of, and I leave with a full understanding (and it surprises me that it seems like they performed surgery at all without providing you enough information to even understand the seriousness of the surgery, but again, I don't know your vet, I don't kow what they did or didn't say, and it's always horribly frightening to be told your pet needs surgery at all, so perhaps you couldn't absorb it all, but again, the vet should be attuned to you and make certain you understand, even in an emergency surgery situation, which this may have been, you don't say, but tumors on or about the back, because of the proximity to the spinal cord and brain stem are always serious and fast track, but their duty is still the same to you to make sure you understand so you can make informed decisions about your pet's care and not have surprises that are frightening like this);
2) Don't leave until you are comfortable that all your questions were answered and you understand what they explained to you
No one is stupid for taking as much time as they need to understand what the vet is trying to explain. There is a reason why those of us who are not vets go to vets, and again, your relationship with your vet is just as important as your relationship with your physician. You are entrusting a special part of your life to someone, and if they in any way make you feel stupid for repeating questions they did not answer sufficiently, that is their problem, not yours, and don't leave until you understand. Any vet who makes you feel in any way rushed or that your questions are stupid should also give you something to think about when you do finish the appointment, because it may be time for you to find a vet with whom you can communicate and who doesn't use tactics to get you out of the office or who is so busy admiring themselves and making decisions for you that they don't realize they are doing harm not just to the animals, but to the owners. And they will unfortunately realize in that case that they are not in fact Gods, but the non-information thing is not acceptable, and the more people who stand their ground and ignore any signals or impatience in this situation will show the vet that they in fact have been at fault all along.
3) Your appointment time is not limited to any specific time, it will take as long as it takes so that you have the informational tools to understand, so that your dog can be fully and properly examined, and all of your questions and follow up questions are answered directly and fully, even if you have to ask them several times to understand. It is not reasonable for a vet to limit the time for appointments like this at all, I know that is an obvious "duh" statement, but I've been surprised at things I've seen with others' vets (I've been lucky, I've known and had the same vet for 27 years, so I am really not joking when I say I would entrust her and her original partner, still there, with MY life) who in the name of efficiency limit your time you get to see them. That works in an office or a seminar, it does not work when every animal is different, and every owner is different. And if you even feel a little ping of guilt that you may be taking too much of your vet's time, just keep this thought in your mind, and even write it somewhere on your notebook where they can't see it, put it in code, I don't know, but think about this - who is getting paid to be there and who is not? They are providing you with a service for which they chose their rate, and that entitles you to their full service, which includes as much of their time as you need to understand what is going on, and that includes a full and complete physical examination and treatment, if necessary, of your dog. Your office visit is charged to you, it is for their time, it is not for medications, it is not for products, you get the privilege of paying for each product separately in addition to the money you pay for their time; and,
4) If you leave with any questions or concerns, get a second opinion Follow your instincts, and if something doesn't sound right after they've explained it to you, get a second opinion at another vet.
The one thing, though, you should never accept is subgrade service that makes you feel stupid, feel like you are wasting their time, taking up their time, or in any way are annoying them, and that includes vets who have emergency services and who end up spending two hours on the phone telling you why they don't have ten minutes to see your pet in an emergency, I write them off immediately, but your relationship with your vet should be open and honest and comfortable. Maybe it will be as simple as talking with your vet openly if you like them and want to keep them. That may end up forging a strong bond between you to work as a team in caring for your pet, it may show you their true nature, but a good vet is one you can talk to without any concerns for time or feeling stupid. We're not vets, it is not reasonable to expect us to know everything a vet knows (and it is not realistic to think that all vets are good ones, either. Someone always graduates last in their class, you know, so it may be that they are insecure and don't want to sound stupid, so don't explain anything to you- that is still unacceptable, but again, consider that as well). So get that follow up appointment, get the information you need, question, question, question, so you could perform the surgery yourself, that is how much you should understand about the procedure, its aftermath, every recovery issue, etc., because you have a duty to your furry family member, but they have a duty to both of you. I will be keeping you in my thoughts and hoping that by now (I noticed the date you posted this) the symptoms are gone. If that is the case, that is absolutely wonderful, and I still think more good can come of this situation because it did open up these other issues with the vet that you can now consider and use to evaluate how this transpired and decide whether it is a relationship that is fine, one that you feel is worth discussing now rather than waiting until another situation arises when you will be stressed out and scared, or looking now for another vet that should have no problem meeting with you and answering general questions you may have, getting a sense of the veterinary practice, the staff, etc.
You are the best judge of this situation, and you should absolutely trust yourself, and know that information is power, and withholding it takes that power away from you. I wish you the best of luck and hope that this was helpful to you
