It can definitely be confusing trying to determine if your baby is sick just from loose stools, especially for new parents. This link can help sort it out.

This article does a good job of explaining the differences in bowel movements in a baby that is breastfed, a baby that is formula fed, and a baby that is just starting solids. I've had children that have went through all three stages and what they describe is pretty accurate to what I've seen in my own children.