I have a birth control pill left but i've started bleeding already? is this normal? it started after i had sex yesterday?

Maybe. Hard to say.: Unexpected vaginal bleeding is common and can be caused by a variety of reasons. Postcoital, ie after sex bleeding, makes me think you may need to get your cervix examined for possible polyp, inflamed/raw ectropion, or traumatized Nabothian cyst. Get speculum exam, Pap ASAP & if concerned, take pregnancy test in about a week.

Post-coital spotting: Spotting & irregular periods are very common in young women. As long as you take your birth control pills as directed at the same time every day (within an hour or so daily), you should be protected from getting pregnant. Spotting is thus just a bother. Post-coital spotting should be evaluated at least once to be sure there's nothing else going on eg injury, infection, abnormal growth/irritation