My newborn has not pooped since i switch to breast milk what should i do?

Probably OK: Breast milk is better digested than formula. At 3-4 wks of age breast fed babies may have a decrease in frequency of poops to every few days. Switching between breast and bottle can contribute to variation in frequency and character of poop. If your baby is eating well, belly is soft, and s/he is not fussy/irritable/crying more than usual it's probably fine. If you are worried, see your doctor.

Wait: Breastfed infants often skip several days between bowel movements. As long as they are well hydrated, feeding well, and not otherwise ill appearing, then simply wait. If the stool is soft when it comes, then your baby might develop a new stool pattern of every few days. Constipation in infants is defined by consistency of stool, not frequency. It should be soft.