Hi
Thanks for writing to the forum!
Many a times an allergy starts in reaction to heat or cold. What you have is probably cholinergic urticaria. Exercise, hot baths, tight clothes, spicy food, emotional stress and fever can all cause this. Once this is identified, avoiding the reason can take care of the problem. Sometimes rapid cooling of the body can prevent an attack. Antihistamines also help.
It is difficult to comment beyond this at this stage. Please let me know if there is any thing else and do keep me posted. Take care!
I've researched Cholergenic Urticaria, and there's two details which don't seem to fit, one when I sweat a lot (I tried flooding) I stop itching outright and I don't have the pin head sized hives. I live in Normandy and I hadn't been drinking enough for a while (people tend to forget to drink when it's cold) Also there's quite a large presence of mothballs (naphtaline) Drinking water helps to cool me down