Sometimes when my newborn spits up my breastmilk the milk looks curdled why?

I have had this happen from time to time. If it happens more often and you suspect that there is a sour tummy involved (colic, arching of the back, crying) talk to your dr about possible reflux or lactose intolerance in your diet... but if this is just a rare thing and the baby does not seem to be bothered by it, I would not worry. You can certainly put it on your list of things to run past your ped on your next visit.