If the baby is having diarrhea for more than 3 days you should contact the pediatrician. It could be a variety of minor gastrointestinal bugs that are out there and are easily treatable.
If the baby has soft stools,he should have to eat things which are solid, sometimes having liquid food like lots of milk or milk like things make the stomach upset and causes soft stools.
There is nothing to worry if there is no constipation. You may be giving liquid food to the baby. When baby start changing food habbits, she will automatically become alright.