Here is an article I found.
http://www.livestrong.com/article/75...fter-exercise/
Does it happen only after very strenous workouts? You may be putting too much stress on your body. Also, are you eating sufficiently?
The first thing I would do if I were you is make sure I wasn't pregnant. After that, if you have an Ob-gyn you see regularly you can call and ask to speak to a nurse and ask them about the bleeding. If they think its something serious they may have the doctor call you...you might not even have to go in.