What Causes Light Menstrual Bleeding After Having Sex?

hi i am 18 yrs old and i had sex with my 16yrs old girlfriend on 18th feb.after 5 days i.e. on 23th feb she experienced vaginal bleeding which was lighter than normal periods and lasted for 5 days..she got her last periods on 27rd feb. is she pregnant or is it a side effect of ipill .n we had sex again on 5 march without any contraceptive,i m worried if she can have pregnancy please answer my queries as im very tensed and im nt even able to concentrate on my study for my exams which are taking place.please comment....