Does my corgi have cancer?

My 7 year old corgimix has had some random enlarged lumps on his underbelly and sometimes they go away and then come back or one will disappear and then reappear somewhere else. They are pretty hard lumps. I tapped it and my dog showed no sign of pain whatsoever. I rubbed his tummy and he showed no signs of discomfort. We took him to the vet and they said that they were just fat deposits and not cancer. However since they started appearing (about a year ago) my dog has showed some stiffness in his back hind legs and he seems to get tired faster. I don't know if its because he's out of shape or because the neighborhood vet had a wrong diagnosis and my baby could have cancer. I'm very upset and taking him to another vet just to make sure. But from what I've told you, what do you think? And please no "smart" or "sy" comments. My dog means a lot to me