Originally Posted by Shimagnolo

That happened to me once, and googling turned up "miliaria rubra".

I was descending from Echo Lake to Bergen Park and it was getting warm *fast* due to the time of day *and* because I was descending. I didn't pull off the heavy tights I was wearing over my shorts until I reached Bergen Park, and my legs were covered in a red rash, but were not painful. It took several days to go away.