Hives and heat rash are similar but different.  Hives are a viral infection - they're often welts that flare up from time to time.  The thing that triggers then can be food or stress.  The best way to find out what triggers a flare-up is doing the elimination diet and slowly removing and adding foods to the diet, one ingredient at a time.

Heat rashes are cause when the skin is exposed to very hot climates and the body cannot cool down enough.  The best thing to do is try remove yourself from the hot climate or add cooling ointments like menthol.