I am a white, 42 year old male. My skin, specifically my chest region, turns splotchy red after a hot shower or strenuous activity. What causes this?

Hives. This in temporary it is a release of histamine after exercise or pressure from a hot shower. An oral over the counter no tiring antihistamine such as claritin, (loratadine) allegro, or zyrtc could suppress the reaction.