What Causes Vaginal Bleeding After Having Sex While On I-pill?

hello doctor , i am 18 yrs old n my girlfriend is 16 yrs old. she got her last periods on 3rd feb and we had intercourse on 10th feb. due to protection failure she took ipill after 32 hours. on 17th feb i.e 7 days after sex she got vaginal bleeding hich was light n lasted for 3 days. is she pregnant or is it side effects of ipill? please answer my query!