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

I had sex vth ny gf in December and January both the times I used i-pill and the periods are in right time Bt were as In February she missed and the gap is abt 45 days f last period and v evn used preganancy kit where it showed negative aftr few days aftr sex v r tottaly worried tht she is conceived what would b the reason for missed period,Is she conceived?