Bleeding During Lacrosse?!?

Okay so I am in this lacrosse camp (I am 15 and a girl) and everyday we have practice then scrimmages. This camp started two weeks ago and goes on until like August. But anyways, today during the game I was defending this girl and she goes to shoot, so I step in to try to block it. I honestly don't even know what happened, but I think that I got pushed because I fell on the ground and into the goal post. It hurt SO badly and I was so scared I couldn't even talk. The two coaches (Both girls, about 23 & 25) came running over and like tried to get me to sit up and talk. My nose was bleeding like crazy but other than that I thought I was fine. So I go to the sideline, and the game starts again. Then out of no where my underwear feel all wet. I look down and theres blood all over my shorts. So I go to my coach "Kelly, I'm bleeding..." and she was like "Do you have your period...?" and I go "No!" And then like my other coach starts freaking out and called my sister to come pick me up. It totally stopped when I got home though. What on earth was that?!?!?