C.W.
I had a boxer mix when I was a vet tech and then went into medical sales. We were able to work on our own pets during lunch and while I was doing a dental on my Jill I found something that looked like as kind tag in her under arm. I sent it in for biopsy and it was a mast cell tumor...very common in the breed.
This wasn't the end of her, several years later she had a hemangiosacroma in her heart and spleen. It was going to burst very soon and I didn't want that at home so we euthanized her that day.
I would definitely get it checked out, they spread. So thankful my boyfriend is a vet.
Good luck!