All four of my pregnancies I've broken out in hives once during the pregnancy without any rhyme or reason. I always take a cold shower and a dose of benedryl and resist the urge to scratch until they go down. If you're like me the more you scratch the bigger they'll get, the longer they'll last, and the more will pop up so don't scratch! Each pregnancy it's only happened once during and I've never figured out why.
Are you taking any different meds and have you tried benadryl ich cream it should be safe to use if not I would call your doc too just to be sure