There's no guarantee it's the food, could be environmental too. I know lots of bullies had horrible allergies this summer.
Also, I would give each formula 30 days (not including the week or so it takes to switch) to see if it's really the food or something else.
How do you know the hives were caused by the Pork n' Peas formula? Sure, it could of been the food, but there's also a high probability it was caused by something else.