As long as the biopsy came back benign, I wouldn't worry about it.
Could be her body's reaction to one of the injections, ( outside induced bacteria ) her body is merely protecting against, or just a lipoma. Lipomas are harmless fatty deposits many older dogs get, in any area of their bodies.
Our 14 year old lab mix has a new lump or bump every week it seems like. Most lipomas if removed, usually come back, so it is a lot to put a dog through to have them removed, especially with possibility of returning.
Our pup made the mistake of chasing a campground cat when she was about 6 years old. Within a couple of months, she developed a bump on the side of her head by her ear. We took her into the vet, who proceeded to tell us it was probably a mast cell tumor, and she most likely didnt have much longer to live. That was 8 years ago. The bump has changed over the years, gotten bigger, and smaller, now is just stable.
I think the cat smacked her and lost a claw in her hear, and her body just formed a fibroma around it as a protective measure.
Just keep an eye on it and watch for ulcerations around or on it. If it stays smooth, I wouldn't worry. Dog's bodies, just like human bodies, have the amazing ability to heal themselves.
Best of luck!
Cheryl
