I seem to get very itchy all over when my body feels too hot. what could be the cause of this?

Heat rash: Sometimes the vasodilation associated with warmth (flushing) can lead to a condition called heat rash, which is caused by release of histamine from the mast cells in the skin. This causes a itchy, raised rash, sometimes with welts. Symptoms can be controlled with antihistamines such as Benadryl (diphenhydramine). Talk to your doctor if this is troublesome for you.