When my baby was a newborn he was on breast milk and formula. He didn't go for 4 days and I got all worried and phoned a helpline. Apparently, this is quite normal, especially in breastfed babies. As long as your baby seems happy and is feeding normally, it should be ok. It is also highly unlikely that your baby is constipated if she is on breast milk. Give it a couple more days and if there is still no action, give your child health nurse a call to put your mind at rest.