Ready to feed formula. Could this be the problem?

My son is on WIC and gets the concentrated formula that you mix with water, well his formula does not last the month so I have been buying the ready to feed formula. It seems like every month when I give him the ready to feed he gets diareah bad around this time..Has anyone else had this problem with their baby? Im just wondering if the ready to feed has something in it that some babies cannot process properly