does she seem to be straining? if she doesn't seem like she is straining to go or pushing, etc she might not be constipated.. some babies will go days without going.
If she does seem like she needs to go see if she will drink some water, if not (alot of babies dont like water) get some apple prune flavored baby juice and do like 2oz of water mixed with 1oz of baby juice, or 2oz of water mixed with 2oz of juice; depending on how much you want to give her.
Colton was constipated alot, and still gets that way sometimes. I give him juice everyday and have since he was two weeks old. I've always done half and half with juice and water.