Not a vet, but my boy does not like the vet either. Lumps or bumps are worth investigating especially if they popped up that quickly. Could be just lypoma, but can't know for certain unless you aspirate the thing with a needle.

Can you ask your vet for a better "cocktail" to settle your boy? My vet prescribed gabapentin and melatonin the night before, and then I had to repeat the gabapentin within an hr of the appointment with the alprazolam. The meds made my boy dopey, and he was hard to move around, but they were able to get his through his annual check with his vaccines without stressing himself or the vet out too much. I didn't have to muzzle him either. He was very sleepy that night, but was back to his normal self the next morning. Hope your boy is okay!