Sounds like your dog needs some socializing. It is natural for dogs to sniff each others butts to identify each other and to say hi. If your dog is not aroung other dogs she may not understand what they are trying to do and this could cause her to become afraid, maybe that is why she is growling, she doesn't know how to respond to another dog. Dog training courses are very good for socializing. She may not need training but getting acquainted with other dogs would be helpful.