Amenorrhea?

That's just not having your period for 3 consecutive months. And at 17, I have it, but I don't know why!! I'm young, active, eat well (and am at a good weight), and it's almost time for summer so my stress levels are pretty low. However, my sleeping habits are changing due to a new job I got. In fact, they stopped right around the time I got my job, but everywhere I looked online didn't really say anything about your sleeping habits really causing your periods to stop. Also, I took a pregnancy test, and it was negative. Help!!