What Causes Bleeding After Taking I Pill?

me n my girlfriend had sex on 22nd of august, which was 5th day of her period (her period came on 17th august). while having sex condom broke n i ejaculated inside her. So she took i pill the next. after about 7 days i.e. on 30th august she bleeded which continued for 6 days. so now we r confused whether her period should come around 17 or around 30 th. she had done pregnancy test continuously after every week n it is negative. so should we consider that after taking pill her period again started on 30th. pls help..