Feeling run down and tired

Lately I've been feeling like I'm catching a cold and I went to the doctors last Friday and they said my lymph nodes were fighting off some virus. I still feel icky and my period is 10 days late, but it's never on the same time. I'm not pregnant because I'm no sexually active. I'm only 15, almost 16. I eat super healthy and I train for triathlons. Maybe because I'm stressed over school sometimes or its because I don't get enough sleep? I go to bed around 9 pm and wake up around 6:30 for school days. What do you think is wrong?