When I first started getting Crohn's problems about
20 years ago, I would break out in hives every time I got hot -- a warm day, a hot shower, exercise, being in the sun, anything that warmed me up. To this day I still get itchy when I get warm, but I don't get covered in hives like I used to. For me the condition is called cholinergic urticaria.
Hives are fairly common, for everybody, not just Crohnies. It's often difficult to figure out what's causing them. You're thinking along the right lines by considering new medications, new foods, new soaps, detergents, etc. You should probably let your general doctor know, and maybe consider seeing an allergy specialist if this continues or gets worse.
One other thing: Benadryl is an excellent antihistamine and effective for hives. But it has the drawback of making you pretty drowsy, not to mention it only works 4-6 hours. Zyrtec is also considered really good for hives and is now over the counter, and it's available as generic OTC as well (cheaper). It doesn't make you quite as drowsy as benadryl and it's good for a full 24 hours. My allergy doctor recommended zyrtec for me when I have trouble with hives and it seems to work pretty well.