Haha, actually thank you for posting this. It happened to my horse a few months ago and even after it went away I was still wondering exactly what it was. Nice to know it happens to other people (not in a mean way of course).
The vet couldn't be sure but she said it was nothing to really stress about. She had a list of possibilities, and she said since it was directly beneath the girth, it could just be swelling due to some sort of bacterial infection or something of the like. She said it might also be as benign as a simple goose-egg you might get if someone hit you really hard.
She said the only way to know for sure would be for me to monitor it over the next week and tell her everything I observe.
Well, it didn't matter because I cold hosed it everyday after riding and it wen't away before the week was up.
Not saying you should put it out of mind though. I'd start by calling your vet. They might have an answer for you over the phone if they've dealt with it before.