If you eat a lot but still seem to be losing weight Does that mean you have a high metabolism?

I am a teenager and I actually do eat alot. But, I seem to be getting skinnier and skinnier. I used to be a size one about mid-year of 2011. Now, I am a size 00. Does this mean that I have a high metabolism?

I don't really want to see my family doctor, or go to the hospital to get this checked out because I am scared incase it is something else. Just thought I would get someone's point of view.