Your body overheats when body heat can not dissipate faster than your body's core produces it. Did you know that when exercising your body can create 15X more heat than at rest? Heat sensitivity occurs from the pace of generation, environmental factors, or both.
Simply put, your heart moves heat out to your skin, causing sweat, and as your sweat evaporates the result is cooling the skin. A higher temperature or humidity can inhibit the evaporation process, causing your body's core temperature to rise quickly.