yes, it could be just an itch. Or it could be worms. It is probably that the anal glands need to be emptied. All dogs have anal glands that give off some secretions. This is why dogs sniff each others' behinds to say hello. They can tell alot by the smell of the dog. However, many dogs, especially small dogs, can get to a point where the anal glands get too full and they need to be squeezed out. This is why the dog rubs her behind on the flooor, it is uncomfortable for her.
You should call your vet or local grooming salon. Either place should do it. It costs very little money. You just go in and drop your dog off and about 10 minutes later you have a much happier and more comfortable dog back in your arms.
Small dogs get this problem much more often than large dogs. Some small dogs need their glands expelled about once a month. Some large dogs need it only once a year or less. It really depends on the individual dog. If your dog is often scooting her butt across the floor, bring her to the groomer or vet right away! You (and your dog) will be really glad you did!