When I scratch or bump my skin, on any part of my body, a raised hot white itchy rash develops. What could cause this?

Hypersensitive skin. This could represent dermographism, where you can cause welts on your skin with pressure, like writing. Physical urticaria, or hives, also possible. I usually have someone try an oral anti-histamine for this problem.

Related Questions

Prickly itchy rash shows raised bumps, when scratch swell into raised strips. Lasts few mins. Affects diff area of body/hair & diff time. No reason.

Dermatographism. If your rash is a flat one and scratching it causing raised strip, you have dermatographism meaning that your skin is oversensitive. If it is bothersome, taking an otc antihistamine such as zyrtec (cetirizine) will likely control it. Else it is not a major health issue. As always, mention this to your doctor when you see him/her next time.