Is There Any Connection Of Serious Diarrhea And A Switch

Is there any connection of serious diarrhea and a switch from breastmilk to cows milk? Our baby's bottom was fantastically free of diaper rash, but when she went from breastmilk to cows milk, we have really fought trying to keep her rash free. To the point of large welts. Unfortunatly she got 8 teeth by the time she was a year old, and started on solid food. Lots of it! Should we avoid high acid fruits, too?