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

I usually have period for 2-3 days at maximum.. I had last period on 19th December 2014 which lasted up to 21st December , then I had multiple sex with my bf on 23rd. Then I took one ipill within 17 hours of our first act of sex. Now today 25th but I didn t had my period yet.. What does it mean ? M I pregnant ? I had used ipills but they always work on time. Plz kindly reply