I get hives regularly but it's due to my allergies, not my Crohns. Have you seen an allergist? Had allergy testing done? Hives are really common but pretty easy to control.
I take a Zyrtec a day just for hives. It does nothing else for my allergies but if I don't take it, I break out all over my body. Before I saw an allergist about them, I was taking Benadryl like candy to get rid of them which makes me pretty loopy. If the Zyrtec is working, I'd assume it's a basic allergic reaction to something possibly environmental.