What Causes A Delayed Period Since Taking Postinor-2 Tablets?

I had My Last Mens. period at 28 september. Took Postinor-2 twice(two tablets each time) at 2nd october and another at 20 november(23 days of my lmp).. Now its december 3 i didnt have my period.. its 7-8 days late... took home pregnancy test today and came out negative.. What is you advice ?