Talk to Gynecologist on Missed Period

Age 29, 14 Weeks postpartum Two C.Sections, No PCOS, Never been on contraceptives, Regular 28 days menstrual Cycle, Motherfeed only in night Practicing successfully Withdrawal method after both kids Lmp 13 February, expected next period Today 12th March 7th march, dark brown/black discharge on tissue paper when wiped, 8th march, no blood flow on pad, but same dark brownish discharge on pad and tissue along with mild cramps and low backache. Remained 1 day Afte that clear light yellow/white discharge Today is 29th day but no sign of period. Why is that? What does it mean? Kindly guide PS: intercourse on 5th,9th,17th,23rd and 28th day.