What Causes Severe Bleeding After A Week Of Taking I Pill?

hi me and my bf had unprotected sex on 17 and with in 6 hours i had an i-pill and a week later on 24 th i started bleeding for 2 days ,but as per my period cycle its should start on 6th of every month, now i am feeling tried and week .what might be the chance of me getting pregnancy and if so how can i avoid it .plz help me out