Don't usually post on this board but just to say, my youngest did this when he was a baby. Can't remember how old but he was little, only having breast milk at that point.
He went 8/9 days without going, I was frantic but eventually he went and he needed a bath afterwards!! It went all the way up his back into his hair!!
Take your baby to the doc if you're worried. I did twice during the 'no poo zone' I think and they could not find anything wrong. The health visitor told me that sometimes BF baby don't produce much waste if the milk is very good quality, they just absorb it all and don't need to poo. I recall wanting to slap her at the time and felt utterly fobbed off, but just thought I would pass it on!
Try not to worry but get it checked out to be on the safe side.