Added to my original post: Just want to mention here that it's not necessary to give formula to babies over a year old. They don't need formula anymore. They need food instead. If they are still tied to bottles, transitioning them to a cup will help.
The ped should be telling you how many ounces of milk a day they should be getting, and how much food to give.
I agree with the poster that you don't mix milk and formula (mothers mix milk with breast milk to help them transition.
I would write down every food that they are fed. Don't offer them new foods at this point. You could keep giving them formula while you wait to see if the food they've been eating is the problem.
Also, never give babies under 2 years old 2% milk. Babies need FAT for development. The pediatrician will tell you that. Whole milk ONLY.
Original:
More likely a new food. Talk to your ped if Desitin doesn't work.