I used to work in a vet's office, for several months. The most common thing she did was spays, nueters, and fatty tumor removal. Most likely the dog just has a fatty tumor. However, it could be something more serious of course.
We had one dog, and doberman, it had, like, 15 fatty tumors she removed, finally the vet (jokingly) told us to stop looking for them, LOL.