I second what Tulip said. You aren't eating enough, and your recent issues with hair loss, mood swings and the like could be signs that you also don't weigh enough. How tall are you? Have other people made comments about how thin you've gotten? Do you find yourself defending your weight and size with friends and family members? Do you consciously exercise more if you feel like you've cheated on your diet?
From the time I was 14 until I was well into my 20s, I suffered from what I would call disordered eating. I wasn't technically thin enough to be an anorexic--although I was pretty darn thin at times--but my fixation on food intake, a distorted view of my body and size, and a reliance on diet and exercise to control stressful events all indicate that I had an eating disorder. I so wish that I'd sought help when I was your age and learned how to deal with the emotional issues in my life and family in a healthier way and to see food and exercise as positives rather than things that had to be controlled.
I urge you to talk to a counselor, a trusted friend or family member, or your family doctor about how you're feeling about food and your body. Short of that, please talk to a nutritionalist about how to maintain a healthy, balanced diet that has enough calories in it to sustain your body and activities.
Hugs,
K-
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher