What Causes Goose Bumps And Rash On The Arms And Forehead?

I am 17 years old i am 5'4 and i weight 130 and i tend to sweat a lot and be out in the sun for at least 3 hours a day. I get this tiny looking like goose bumps but its like a rash on my arms and forehead. I put cold water and ice but its spreading a lot. What should i do!?