Why am i losing weight when i'm eating more fattening food?

Possibilities: There are multiple possibilities here. For a 16 year old girl, the main one would be eating less calories than you're expending through exercise, etc. Hyperthyroidism is another possibility, and difficulty absorbing nutrients would be another. These are not the only ones. Depending on the extent of weight loss, your best option would be to see your physician for an evaluation.