I had the same condition for the longest time and wasn't able to figure it out. Finally, I found a doctor who diagnosed me with cholinergic urticaria
, or urticaria type cholinergic. While there is no real cure, as in my case it sometimes go away on it own. Treatment includes exercise to bring your body temperature up so you first experience it, and then it subsides. I also found antihistamines such as benadryl
lessens the severity of the condition.
Hope this helps!