I have this! If you ever want to talk about it, pm me. (hopefully this comment works)  Edit: since the comment worked.  I started getting hives when I was 15 (now 22) and they're from overheating/over exertion. I didn't know there was anything online about them! I looked them up the first time it happened and found literally nothing.  Since then, I have been "testing" theories I have on them, using myself, and have compiled lots of trial and error data. If you're interested, we should swap stories. Maybe you know something I don't, and vice versa!