Weight Proble?

Hi... im 13 and 3/4 years old and im 93 pounds and im 5'5. Most people at this age are like 120+. I'm average height but not weight. One of the main reasons for this is i'm pretty athletic. I got my black-belt in Taekwon-do and have recently started Brazilian Jiu-jitsu. I was on the basketball team and badminton team this year. 3 times a week I weight train and run three miles on my tredmill with high incline. I just dont know what to do... its not like im that scrawny or weak.. in fact i'm way stronger than those people who weigh like 140 poounds. I eat alot and I eat healthy but yeah can somebody just help me?