I get the same thing. Red bumps/rash on the top of my thighs after a long hot day in the sun. Heat rash. As far as I know, it's just a reaction in the skin to being overheated. Not really something to worry about. It goes away by itself, doesn't hurt or itch so why put drugs on it? And I'd stay away from the cortisone creme. Steroids are not something you want to take any more than absolutely necessary. For me, all it takes is to try cooling off regularly throughout the day. And if I do get it, put a cold towel on when I get home to cool the skin down again. Then just let it clear up by itself. Usually it's gone in a day, two at the most if it's really bad.