I'd go to urgent care.
I've never heard of people reacting to the surgery like that, but some people are allergic to certain meds or to the surgical glue or tape they use. Are you taking ursodiol? (the med some surgeons put their patients on for a few weeks to help prevent gallstones). Hives can be a side effect of that (not a common one, but I know it's a side effect because I was one of those people who got hives from it). Or it could be some other med you're reacting to. Just check with your PCP or go to urgent care. I reacted to either the surgical tape or glue when I had plastic surgery (although I didn't react to it with my RNY, oddly - but maybe it was different brand of tape/glue) - they put me on prednisone for a few days so I'd quit reacting to it. With the ursodiol I was taking after my RNY, the PA at my bariatric clinic just had me quit taking it.
anyway, I doubt it was the surgery per se - but it could be some med or something that you're reacting to. They'll know what to do.