my 16 year old son is training for a triathlon as he has for the past 5 years. this past week, he started developing a rash over his body trunk during exercise as he heats up. this has happened when he cycles and runs. today he was doing interval training on the bicycle and had to stop due to a red raised rash over his trunk, front and back as well as his groin and thighs. today is the first time the rash has spread to his groin and thighs. i give him 25mg benadryl and the rash is gone within 30 min after exercise is over. today is much worse. he had to stop after an hour and 1/2 and take 50 of benadryl. what could this be?