It could be that your baby is lactose intolerant, but I would talk to your doc about it to be sure. I was breastfeeding my little boy and was giving him about 2 oz. of good start a day, but at 4 weeks old, I upped it to 4-6 oz. He started breaking out in a rash all over his body, was fussy, had trouble sleeping, was vomiting and had diarrhea. I panicked and switched to enfamil and nothing got better (not a good idea). I had him tested and sure enough, he was lactose intolerant.