Oops, I forgot to answer your question!

I don't remember exactly how I figured it out. Process of elimination, I think. I did a lot of online research, but I could never find any descriptions that matched my issue. I was going to make an appointment with a dermatologist when I had a bit of a 'breakthrough'.

I rode a century last August that I was expecting to be VERY hot (it's the reason I was training in the heat). It turned out to be the coldest day of the summer (and raining for the first 50 miles) and lo-and-behold...100 miles later in the same shorts I always wear and I got no rash. After that, I made a point of paying attention to my body as it started to heat up and I got to a point where I could almost pinpoint when it was occuring by how I felt. Then I did experiments with long hot rides but using a LOT of water for cooling and sure enough, I could keep it from happening.