Weird lump on dog

A couple of weeks ago I found a lump on my dog. It is by her shoulder -- not the shoulder blade but where the front paw seems to move (the shoulder socket?). It appeared suddenly, or at least I'm almost certain. My dog is a massage addict, so I tend to spend MUCH time rubbing, petting, and loving her daily. I would have noticed it if it was slowly forming.

I panicked at first. Then I remembered about a couple months before, my mom's dog had the same thing happen in almost the same place. She noticed his after my sister's dog had been to visit them. The 2 dogs play pretty rough, so she figured it was a knot from Lucy biting Max when they were playing. It kept reducing in size and went away after about a month. The vet figured that was what it was from. When I noticed Haley's, it was a couple days after a dog up the street thought she would be tough and try to bully Haley while we were walking. Teenie (yes, that was the dog's name) ran up to us like she was going to play -- they've played in the past. Then she started growling and snarling and "pushing" Haley. Now, Haley LOVES pretty much all dogs and is happy go lucky while on walks, but she also has a mean streak in her when it comes to protecting bones, toys, me, etc. I guess she got pretty ticked when Teenie did that and started to fight back. Teenie had the funniest look of shock on her face (Haley wasn't biting into her, just snarling and snipping). Teenie ran away and hasn't bothered us since.

Anyhow, I'm wondering if it could possibly be a knot in the muscle or something from being pinched during a bite. I think it's starting to get smaller now. I was going to give it a couple weeks to see if the size decreased or not before going to the vet to have her opinion or get it biopsied. Has anyone else seen this happen before? Do I have anything to worry about? I will keep checking it to make sure it is indeed decreasing in size. Oh, I have pushed, prodded, and put pressure on it to see if it hurts Haley now, and she doesn't react. She did a little after I first noticed it, but now it doesn't bother her at all.