There's a reason that doctors don't want to weigh babies every day. They know that weight gain isn't always constant, the way those nice smooth growth curves would seem to imply. Growth proceeds in spurts, and it's very common for a baby to pause in one dimension while spurting up in another. Often a baby will pause in weight while growing significantly in height, or vice versa.
After 6 weeks, it's normal for a baby to go as much as a week between poops. Just be prepared for a diaper-buster when the poop finally does arrive.