When I'm moving fast for short periods of time lets say sprints I get a rash all over my legs and abdominal area. It's so itchy and it burns like crazy. I've been asking my doctor friends and finally I have an answer.

"Exercise urticaria, sometimes mistaken as exercise allergy, itchy legs, itchy legs syndrome or itchy pants syndrome, is a form of urticaria that happens during exercise. It is characterized by itching, swelling or hives on the legs, arms, torso or neck during or after exercise.
Cholinergic urticaria is brought on by a physical stimulus. Although the physical stimulus might be considered to be sweat, the actual precipitating cause is increased body temperature. Lesions usually appear within a few minutes of sweating, and may last for 30 minutes to over an hour.
In extreme cases, the condition can progress to an anaphylactic reaction, causing the person to pass out or suffocate due to blocked airways. People suffering from cholinergic urticaria may carry an epinephrine autoinjector (such as an EpiPen) in case of anaphylaxis."

http://familydoctor.org/online/famdocen/home/healthy/physical/injuries/737.html

Ever since High school I've gotten this. It's a pain in the ass. LOL! There are times on a run or ride or mid WOD where I want to rip my pants off and scratch the living shit out of my legs and hips. The welts I get are huge but only last about 45mins or so. If it gets really bad my face can even break out. Overall it happens mostly while running, it can really make me crazy. What I mean by that is that it's painful and I'll have to stop.
Our bodies are strange and amazing at the same time!