Rash on Legs

Ever since riding a recumbent bike where my thighs face up while pedaling, I have developed a rash when it's warm and sunny. My thighs break out in a red slightly raised blotchy rash. It's not very itchy but sensitive to touch. Whenever I get this rash, I also get slight chills and a feeling of alternating between hot and cold.
Before riding, I apply a full spectrum sun screen but it doesn't seem to help. A perplexing part of this problem is that if I wear loose fitting shorts or longer pants, I still get the rash when it's warm and sunny. The rash always goes away in a few days.  
Any suggestions would be most helpful.

Thank you,
Joy

[Alan Rockoff, MD]

I apologize, but your description doesn't match any syndromes with which I'm familiar.  My usual advice when people get rashes which don't correspond to anything in particular (a not-uncommon occurrence) is to treat the symptoms (moisturize, apply OTC Sarna lotion for itch) and not be concerned with larger implications (like sun damage), of which there aren't likely to be any.  If the rash spreads, see a docotr.  If it goes away, forget about it.

Best.

Dr. Rockoff