Over the summer I was getting a patchy red rash under both my wool socks and my (non-wool) sports bra, only after long runs. Shorter runs wouldn't trigger it, long periods of wearing the same things on the bike or just around wouldn't trigger it. It would go away within 24 hours.

I'm kind of guessing it was a reaction to my laundry detergent (clear and unscented, but still a commercial detergent). No real idea. I'm just thankful it didn't itch - I wouldn't even have known it was there if I hadn't seen it.