What Causes Delay In Menstruation Other Than Pregnancy?

My last periods was started on 26th December.l am too much healthy(10kg more than the height-weight ratio).so that I am going through irregular periods. In the month of January we had unprotected sex for several times but the semens was ejected outside.i have already taken modus 10mg from 1to 5th February twice a day. As I am not having the periods still.so that I have tested pregnency test for last two days.it also shows negative.
What is the reason for not having periods. Is it possible to me becoming pregnant though showing negative.