switched formulas from regular enfamila to nutramigen due to constipation.

My sons 4 weeks today and we recently had a constipation problem last week the dr. Switched him from the regular enfamil to nutramigen due to his dad being lactose and with me milk gives me gas, so gentlease was out of the question she said. But hes been drinking the nutramigen good its just that it seeems like every diaper is a poopy diaper and its been a week since we switched formulas. What im asking is has anybody else had this issue, or is this normal?