Uni has had several bumps removed and biopsied, and thankfully they were all nothing. She had one on her foot that started small, then got big and turned black, then eventually fell off!
But they elected not for surgical removal bc there wasn't much skin there to stitch it back up. She had another on her thigh, that the vet removed in the office with a sharp pokey thing. That turned out to be a follicular cyst. It was deep in the skin and quite imbedded. She had another on her elbow that was small and grew large, and then fell off. Really those things can be anything. I say get them to do a needle aspiration in the office, no anesthesia, have them send the cells off to a lab and find out what it is. If it's nothing, then don't worry. Yes it will cost about $100 to find out it could be nothing at all, but IMO it's worth the piece of mind. I've done it I think three or four times now.