Sounds like hives/allergic reaction to me too (although I wouldn't totally rule out stress). My daughter has allergies which show as headaches, nausea, itches, various rashes/hives/welts, for her the main problem (even airborn!!!!) is any contact with any sort of petroleum based products, perfumes, cleansers, also alot of furniture is finished off with a petroleum based product (we discovered her allergy when we bought her new solid wood bedroom furniture!!)
so don't just think in terms of cleansers, soaps, perfumes, also think are you aranyone you live with using anything new or has there been a new purchase. Since it first happened at work in the morning though it would be worth thinking what happened that morning? Did you come in contact with anything new, buy anything new? Did work buy anything new or start using something new? It might only come out at night after building up during the day????
In terms of stress, just in case, try some relaxation exercises (this could also help the hives!!).