I had unprotected sex with a guy about six years ago and he didn't cum inside me but ony stomach and i got a rash there i can't remember how long after that. but now i break out in this itch and hiives whenever i get i'm heat and take hot showers or any

Not related: I can not think of any reason where the exposure to semen would lead to this kind of reaction 6 years later. On the other hand you should not be getting a rash and hives when exposed to heat. I will refer this question to dermatology and allergy and maybe someone from that specialty will weigh in.

Cholinergic hives: Thanks dr. Livingston. Hives that occur when you get hot, sweat or take a hot shower are called cholinergic urticaria, a form of physical urticaria. Other causes of physical urticaria include pressure, release of pressure, vibration, cold, and sun. Hopefully you are not still having unprotected intercourse & you have been tested for all sexually-transmitted diseases. But the hives are unrelated.

Not related: I don't see how this could be related to the sex. You may have urticaria now, many causes. Sort through it with a dermatologist.