What Causes Periods After Four Days Of Taking I Pill?

I'm 26.I had done my 1st intercourse with my partner.with out any protection.we dont know wheather sparm had entered with me or not. after 24hrs I had I-pill. after 4 days I had faced my periods, which was not as per schedule. it was 14 day from last periods. plz suggest me now what to do?