Causes of prickly heat rash

• Heat rash begins with excessive perspiration, usually in a hot, humid environment. 
• The perspiration makes it easier for dead skin cells and bacteria on the skin to block the sweat glands, forming a barrier and trapping sweat beneath the skin, where it builds up, causing the characteristic bumps. 
• As the bumps burst and sweat is released, there may be a prickly, or stinging sensation that gives this condition its name.

Reference: webMD