Why Am I Still Missing My Period, Despite Ovulating And Testing Negative For Pregnancy, Following Unprotected Sex And Using A Post-pill?

I gave birth last year November 24 but breatfeed for one month so this year March I had unprotected sex and I use postpill after I saw my first menses that same month two times (early and late of March) so since then I didn t have sex but now last month I didn t see my menses only what I see is dropping of blood whereas in the absence of menses I am ovulating and tested negative for pregnancy test so why is my menses still missing