Other Causes of Rashes. 1  Heat rash. Heat can bring urticaria out, or make it worse. 2  Chronic 'idiopathic' urticaria. By far the most common form of urticaria, which lasts a long time. 3  Cold Urticaria. If cold brings out your hives, take great care to avoid falling in the water, and see a specialist.  Pressure Urticaria.