The same thing happened to my daughter this summer. She had lunch and within an hour, her stomach and back were completely covered in hives. It crept up her neck and face by dinner. In the morning her legs and arms were covered. Then it reversed itself, disappearing from her midsection, then face, then legs.
The hives came back briefly during dinner that night, but were gone permanently the next morning. The pediatrician had no clue what caused it -- we hadn't changed soaps or shampoos or introduced new foods or medicines. At our next gastro appt I asked if, now that she's taking creon, foods that she hadn't been sensitive to would now cause a reaction. He really didn't think so. She had been taking Creon for about five months at that point, so it he'd didn't think the hives were caused by the Creon either.
It was over as mysteriously as it started. We never found out what caused it and it's never come back. I wish I could help you figure out what caused your daughter's reaction... I remember being on edge for days afterwards, nervous about what I may have given to my daughter to bring on that reaction and would I do it again -- or bring on a worse reaction. But thankfully, she was totally okay.
Good luck with this!!! I am curious to see if you do find a cause...