when i was 12-13, i wieghed 175-180lbs, then puberty hit and i just started losing wieght, i went from 175- to 165, then 165 to 155. Excercising and heating healthy are key, but you have to remember your still young, and your body is still gonna go through a lot of changes cuz of puberty. Thats just the way it wuz for me.