What Causes Itchiness And Red Spots In The Skin?

Hi I have a problem where when i get hot its like i m having an alergic reaction my body starts stinging allover and i can rub my fingure across my skin and it leaves a red mark like my body is overloaded with histamine. I take alegra d 24hr and singulair and have switched washing detergent and it seems to continue. It really has an effect on me because i work outside in the sun !