Probably not. Anytime you add new foods to a baby's diet, her poops will change a bit. The loose stools may not be diarrhea at all, but just her body's way of adjusting to the formula. Diarrhea is not only diagnosed by the consistency of a baby's poop, but by how often she has a dirty diaper. A bout of diarrhea would probably double her daily dirty diaper count. So don't jump to conclusions and start trying different formulas right away, since this can prompt a whole new set of tummy troubles.
But if the diarrhea isn't getting better after a week or so, if you notice blood in your baby's stool or if she develops a bad rash, talk to your pediatrician. He will probably want to run a test on your baby's poop (called a Hemoccult) to see whether a milk protein allergy could be the culprit. If an allergy is discovered, your doctor may suggest other formulas, like Enfamil's Nutramigen or Similac's Alimentum, which are made with milk proteins that have already been broken so they're more easily digested and less likely to cause a reaction. He'll also suggest that you eliminate cow's milk and other dairy sources from your diet, since these can be transmitted to your baby through your breast milk.
Copyright 2009 Meredith Corporation.