those lumps are a pain. The vet can poke it and take a sample of what's inside, look at it under the microscope and tell if it's "cancerous" or not in one visit. Grace has one on her stubby that they meant to clip off during her surgery. they forgot, but the vet did the looksie at it when she was in late last month and it's not cancerous. they'll probably remove it soon and they may give me a break on the price since they forgot to get to it on the 4th.
just like with people, dogs grow weird bumps and lumps that serve no purpose and do nothing more than look big.
I had a dog once that got lumps. turns out she was getting pimples and if you squeezed them, they would drain and go away. Lady got one on her paw and before we had the chance to have it removed she licked it and it burst and bled and went away. Most large breeds get those on the lips and toes. Of course, Grace has to be different and get one on her stub (which the vet said was most likely scar tissue from her crop that took almost 4 years to bubble up.)
I would go with his confidence in his thinking it's probably a beneign tumor. If she's not showing signs of pain or strange sickness...the
n i think you're ok not to worry.

keep us posted.