I am 19 years old and I am in great shape. I used to go to the gym about 4 days of week and I was as healthy as could be. Lately though, over the last couple months I have noticed that I am ALWAYS tired and I lack energy. I sleep most nights 9-10 hours, but still I can't function during the day. I feel like a zombie. I'm so tired that I get headaches most days and I lack any motivation to go to the gym, it even affects my appetite. I sometimes just can't eat. What could be causing this?