What could this lump on my dog's shoulder be?

First off: We are going to the vet on Friday. I repeat, we have a Friday appointment at the vet (because, I wanted him to go on a real nice vacation this year lol).

My dog was at the vet three weeks ago, because she was itching. My husband told me, "he gave her a whole bunch of shots", but couldn't tell me where, or how many. (Don't worry, this is going to be relevant).

This past Sunday, I fell down the stairs of our apartment building, I tripped on my dog's leash. Now, I know it may have been selfish of me, but I kind of didn't want to break my neck, so I did NOT see if she fell, or got dragged...all I know is that when I was done falling, she was standing next to me. At that time, I wasn't all that sore, so we continued on with our walk. Monday night, I noticed a lump on her shoulder, probably about the size of a large grape, and it feels like a fluid filled cyst that I have. It doesn't appear to hurt her at all, but it is very near where her harness hits, and if she was dragged or she fell, she would've hit that part, for sure. Everything else about her is completely normal, except that lump. I only talked to the Office Manager at the vet office, so she couldn't tell me whether it was caused by a vaccination or a blow to the area. I don't know if this is relevant, but a few years ago, when my cat strained a muscle in his leg, he also had a lump very similar to hers. Since this isn't considered an emergency, we are going to the vet on Friday. I just wanted to see what everyone thought about it, or if you've had something similar happen.

We're going to the vet on Friday (in case you didn't get that before).

Thanks in advance!