itchy red skin from heart rate increase

I got skin problem due to exercising or heat, for example, like being in a hot shower or out on a hot day, I'll even go for a walk and as soon as my heart rate or body temperature rise, i get red skin all over my body, it appears on my shoulders, neck, chest, stomach, and back, a little on my legs and on my arms i get bumps =[ I neglect to ask my doctor idk why.
I'm 175lbs 6'2 and keep fit, but its hard to exercise or have sex when i get red and start itching all over, it intensifies after my workout when i cool down