This happens sometimes, but not always, to me. (And more on my stomach and chest than anywhere else.) It was really noticeable when I was dealing with chronic hives (for which no cause was ever found)–I would scratch my arm and swell up a bit. I take claritin and singulair now and just generally have less allergy stuff going on. (Though I was trying to exercise yesterday and got really itchy. So crossing fingers that it’s not starting up again.)
I am having a hard time finding information about mast cell disease, aside from a few sources that all say the same basic things and veterinary med sources!
Check out: http://www.mastocytosis.ca/ Hope it helps!