My doctor thinks this is just not watching what i eat...bit idk..?

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! i went to the doctor today and she had my blood drawn. she didnt seem too concerned about the FIFTY POUNDS. she thinks its because my growth spurts are over and i just need to start watching it...but FIFTY POUNDS?! ive never had a growth spurt. I've never been able to NOT watch it. i used to have an eating disorder. does anyone know whats going on?