I've had this problem for few months now and I can't stand it anymore.
Whenever I get a little stressed, mad or anything, it starts itching a little. At first. 10 seconds later my whole body itches like crazy - back, chest, hair, arms and legs. Even my face itches. The weird thing is that it's not a problem when I'm relaxed - only when I get nervous/mad/stressed!
Physical Urticaria are hives brought on by a physical stimulus, I get it from cold weather, water and pressure.
Heat can induce hives (different from "heat rash"), sweat, exercise, sunlight and in the case of emotion-triggered hives - it's probably a histamine reaction to adrenaline - but they don't really know what causes physical urticaria.
It can be treated with anti-histamines. You might want to get a basic blood work up just to rule out other causes like thyroid disease or lupus.