What Causes Delayed Periods During Postpartum?

hi doc i m a mother of one baby he is of four month i got period once after delivery but now i missed my cycle n doc give me primulot bd for 5 days its over 2 days but i still dunt have periods i have done pregnancy strip test which is negative n had a scan it was also negative no sac was found so can u plz tell me when would periods start or do u suggest any medecine