What Causes Vaginal Bleeding Post Intercourse?

hello doctor,
me and my girlfriend had protected sex on july11 of this year. She had also had a birth control pill on the following day. however her periods due on aug 5 were delayed.2 Urine pregnancy tests on the same day and following day showed negative. She still hasnt got her periods. today however she got her periods but they abruptly stopped after 4 hours or so. On reading on the net we came across something called spotting.. We are really worried. please advice