What Causes Vaginal Bleeding After Taking An I-pill?

I got my periods on 20th Dec'10; had sex on 29th. Even though my bf didn't come inside me, I tuk an I-pill within 10 hrs. I started bleeding snc yesterday (started wd bleeding only wen d full bladder wd press against d uterus, or while peeing). Now I feel like it's periods. What's happening??