So the other day, after I finished untacking, I ran a body brush all over my horse to get off some mud. I noticed that on his belly, slightly behind where the girth sits, there were a few large bumps. They looked swollen and painful, so I pressed them. He didn't flinch, so I suppose they didn't hurt him. They felt a little hot, so I cold hosed his belly (he did not enjoy that... But the hose had no hot water). After about a half hour, the swelling seemed to be going down and it wasn't hot anymore. When I came back the next day, it was completely gone, until I rode again, the same thing happened.
I know his girth isn't too tight, and it is in the right place. Does this happen to anyone else's horse? What should I do when it happens? Is it just because of the tightness of the girth restricting blood flow or something?