What Causes Delayed Period After Taking I-pill Followed By Unprotected Sex?

I had an unprotected sex with my partner on 7th night and 8th morning. I have taken an emergency contraceptive pill (i-pill) on 8th evening at about 8pm.

My due date for period was on 13th but till date it has not started. I have done a pregnency test with prega news kit on 15th and it was negative. Now my dates are still not coming and I m worried about my pregenency as I dont want it now. Please help me what shoud i do?