What Causes Delay In Menstruation Other Than Pregnancy?

Hi, My last period date was 25th. I had unprotected sex on 20th of this month and taken i pill on 21st evening. but still my periods have not came. On the other hand, i am going on vaction on 1st -4th April n i dont want my periods to come on the same days n ruin my trip. Please suggest me that is it safe to take primolut n tablet to postpone the periods till 4th April after taking I pill. My pregnency test is -ve