I had a similar occurance back in 2003-04. In mid 03, I was 130 pounds at 5'3" and age 11, which I thought was a little too much (I looked a bit pudgy too). So I started loosing weight, and by the end of August 2004 I was down to 99 pounds at 5'7", which makes a BMI index of 15.5 (that's way underweight). I started gaining after that, though carefully. Now I'm holding steady around 170 at 6'1"ish, which is near normal.