What Causes Vaginal Bleeding After Having Morning After Pill?

Hi, I had sex with my boyfriend on the 29th of July 2010 at 3pm and the condom split so the next day I went and got the morning after pill , 4 days later I started to bleed like my normal period but that wasn t due for at least 2 weeks, I bled for over a week and now it s started again an I don t understand why! Can you help ?