A.C.
It is pretty normal for both breastfed and formula fed babies. Their little digestive tracts don't have the muscle tone to effectively push out their bowel movements. It can also be gas.
Constipation is not the absence of bowel movements necessarily, but straining along with producing a hard, pebbly bm.
There are a number of stomach massage and bicycle leg exercises you can do to help move things along and free trapped gas.
Google "baby gas massage" and you'll get a lot of good tips.