Have you spoken to a doctor about it? More importantly, are you okay? Are you eating normally now? I hope that you're okay and that you can look in that mirror and know that you are absolutely beautiful, just the way you are. Loss of menstruation is really common in athletes who have overworked themselves (on that note, I recommend the book Tiny Pretty Things. It's amazing.), or people who struggle with eating disorders. Please talk to a doctor about it if it's worrying you!