Pressure on a newborn’s scalp during childbirth can damage or rupture tiny blood vessels in the scalp. Blood collects under the scalp, causing a soft bulge or bump to form, typically on the back of the head.Cephalohematoma is often harmless. The bruising or blood buildup should go away without treatment in a couple of weeks or months. * As a baby’s body absorbs the blood from the cephalohematoma, bilirubin levels in the bloodstream can rise, causing jaundice*

It is most likely cephalohematoms. It is not cause of worry and will most likely disappear in s month.

By what you are saying it looks like cephalhematoma.It usually takes 2 to 3 weeks to resolve completely.In case if it is increasing or baby is having jaundice consult Dr immediately.