What Causes Delay In Menstruation Other Than Pregnancy?

The first day of my last period was 26 April. First time I had sex after the period was 19 May. I took 3 tabs daily of primolut n on 27 and 28 May

thinking this will expedite my period. What happened is till today 30 May there is no sign of the period. I made home pregnancy test on 28 May it showed negative. Is there any possibility that I am pregnant ? what is the best way to expedite my period ?