What Causes Lack Of Menstrual Periods After A Child Birth?

Hi Doc, i am Noor ..i have 10 month old son and till today since delivery i haven't got my periods...i had it once after 1 month of delivery for 1 day but not after that. what could be the reason? i also haven't lost any weight since delivery. my eating habits and excersise routine is healthy. why is it the i don't loose weight. i do breastfeed too since day one?