Why am I losing weight without trying?

I'm fifteen and a female. I'm short, ony 5 feet tall. A month and a half ago(roughly) I was about 97 pounds. Now I am only 79. A few weeks ago I was 84.. and I am not trying to lose weight. I don't have an eatin disorder, and I eat at least two meals a day plus a few snacks.. Any ideas? If this continues it cant be good! Im already uderweight..