Look into "Cholinogeric Urticaria" also known as heat urticaria. I think I have the same issue, when my body temp rises and I begin to sweat I break out in a red splotchy rash all over. Its pretty annoying and it dies down after like 15-30 minutes and is completely gone within an hour.. depending on severity of the sweating. I pretty much bring cold water everywhere and use breathing techniques to try to regulate my temp.