Strategies To Lose Weight Fast

I currently weigh 108 pounds at the age of 15, i used to weigh 143 pounds but lost a lot of weight in a very short space of time, around 3 months. I ate very little and now i eat more however still very small amounts. I have been to doctors who say i am underweight and have a low bmi. However, my ankles are still big and i would really like to know how to lose weight on my ankles without having to eat 500 calories a day. Please help!

Please help me! I really would like to get a slimmer body over all!