Very High Weight Gain?

I'm 16, 5'4'', ive run cross country and track for 3 years (going on 4), and eat very healthy. recently, I gained 50 pounds in the last 7 months... I have been running and eating healthier, but my weight still somehow goes up.. In the last week, I've gained 15 pounds. What's happening!? I run miles a day and eat an average of 7-8 servings of vegetables a day. Fiber has no effect on me. I ate a whole bar of ex lax and nothing happened. my hair is starting to fall out and im getting hot and cold flashes. my running times are going up and i cant catch the slowest person. I'm tired ALL THE TIME. can someone tell me what to do? i used to have nightmares that i was this fat... its all out of control!