No period for 10 months?

I'm 16 years old, and I haven't had a period since August of 2010. I am not a virgin but I am obviously not pregnant because 1. I've taken a million pregnancy tests, I don't look or feel pregnant, and I haven't had sex in months. But anyways, I stopped getting my period after I started getting into running, and I know thats probably the reason why I'm not getting it, but I don't know why it's been so long. Since last July I've probably lost 10 lbs but I've been at the same weight for the past like 4 months or so and I feel like my period should have evened out by now right? I'm not super skinny I still have pudge in my stomach area and everything, I'm about 5'1" and weigh arond 95 lbs. What could be wrong and why am I not getting my period still? Is this dangerous? Thanks!