G.♣.
If you've been using the same formula for the past couple of months, I would think that something else is causing the hives.
Did you switch stores? Even though the store brands say "compared to Similac" or "compared to Enfamil," they are not identical. Some of the ingredients are slightly different. Maybe you were using the Walmart brand and recently tried the Meijer brand? If that's the case, it could be the formula.
Either way, I would call your doctor. It could be something else like the bath soap you use or laundry detergent. Your pediatrician can ask you more questions and help you narrow it down. It could be something you never thought could even be an issue.
Don't trust me! Call your pediatrician.