Our Ped said it is common in breastfed babies at that age to go longer between poos. My son was that exact age when he went 11 days with no poo, and I asked his dr about it. She also said that breastmilk is absorbed into the body so much there is little to nothing left to poop out (ahhh love the poop talk!)
I would def talk to your ped if you are concerened though. It is always better to be safe than sorry. Good luck!