I Have Taken Primolute N In End Month Of October.after

I have taken primolute n in end month of October.after then I got my period on 1nov. I had sex on 20th of Nov. And I missed my period which should come on 1dec. I have done pregnancy test through preganews kit as I missed my period. Test was negative. But today is 15th of December but I didn't get my period. What could be the reason.