You'd have to have had a pretty hard fall onto your bike seat to have vaginal bleeding - this means that you might have sustained some internal trauma, eg vaginal or cervical laceration, from foreign body penetration. I'd suggest that you go see your family physician immediately as this isn't at all normal.
One more thing, as a family physician, if you came to me with such a story, I'd want to speak with you alone, eg no partner or parent, and be sure that you hadn't been raped. Given the sad but high prevalence of domestic violence these days, we have to keep an eye out for suspicious stories. And regardless of age, falling on one's bike seat has a low likelihood of causing vaginal bleeding.
In most larger cities in the States, there are rape crisis centers and abuse hotlines that can help immediately remove you from danger while providing you w/emotional support. Good luck!
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