Multiple Factors. There is no one cause for the onset of anorexia nervosa eating disorder. Predisposing factors, including genetic susceptibility, have been shown in research to account for up 60% of cause of onset. Precipitating factors are often related to emotional factors and stressors including trauma. Social and media expectations also receive some blame. Short answer, a combination of biology/environment.
Anorexia. Anorexia is simply loss of appetite, which can come with many conditions including cancer, liver disease, and depression. Anorexia nervosa is a whole other category -- and is the eating disorder with intense fear of gaining weight, refusal to maintain healthy body weight, & distorted body image. It arises from arises from a complex combination of many social, emotional, and biological factors.
"Causes" is correct. A major cause is sexual or emotional trauma. A second significant cause is how eating and weight gain are dealt with in the family. Significant is the inappropriate way "desireable" women are depicted in every aspect of our culture. It distorts girls and women's perceptions of themselves and other women. The problem for men - it distorts how they view and treat daughters, wives and lovers.
Many. Anorexia denotes a decrease in, or lack of both appetite & hunger. This can be caused by to mention a few causes emotional disorder, drugs, nutritional & endocrine deficiencies, anemia, etc. In a person especially a female at your age it is a very serious condition and I would advise you not to delay and get to see your family physician ASAP if you are indeed concerned about anorexia.