Could this be anorexia nervosa?

I don't think so. I have ever been there, and it happened when i was 14, too. I think your daily diet is good enough for a growing teenager. My diet was more extreme--I didn't take any carbs and my calories intake was only about 400 per day. I didn't have my period for about 3.5 mts. I was so frightened but my doctor said it caused by stress and hormonal change in my body. I think nothing's wrong in your diet--the weight loss was caused by the exercises. You've gotta add up some more nutritious food, maybe 2 gles of milk everyday so you will not lose your weight too much. Just be careful when you are exercising--don't let it be overlimit. Hope this help!