newborn not pooping!?

So my baby is 1 week and 4 days old he was breast fed up until he was 5 days old and then I had health complications and had to switch to formula. He was doing great up until last night. He didn't poop at all last night and hasn't so far today. He's eating every 2 hours like he's supposed to and he's had tons of pee diapers. When should I be concerned or contact my doctor?