no poop in 48 hours

My son is 8 weeks tomorrow and it's been 48 hours since he last pooped he is a breast fed baby abc does seam constipated he went from pooping little poops throughout the day to one big poop per day now hasn't pooped in 2 days when he does poop it is musterd color and seedy is that normal or something I should be conserved about he has his 8 week apt Tuesday I was going to ask the doctor then but it's making me nervous