Why is my metabolism to fast?

Im 15 and im possibly the lightest person in my school at 97 pounds. People say i need to eat more but the truth is i can eat more then people think. I EAT ALOT. I eat lots ot junkfood and food in general. Im constantly eating most of the time. I want to gain some weight. I want to gain 10 pounds but it will never happend cuz lets say im 90 pounds then i eat im 91 then as soon as i eat i have to go to the restroom then im 89 pounds >.< why do i not gain any weight!~!!!