I usually get my period every 30 days, exactly. It's very regular. However, this month, it's already two days late, and I don't feel like it's going to come anytime soon. (Usually I get more emotional and my joints hurt and whatnot. Before I was regular, I could always tell a few days before when my period was coming.)
Not only that, but I've been losing weight, and I'm pretty sure you're supposed to gain water weight right before your period. This is the lowest for me, 103lbs at 5'.
Have you recently been sick or has something been stressing you? Maybe you haven't been eating as much for some reason. Do you have a loss of appetite? All of these could be reasons for while your period is late. Any types of slight changes in the body can cause a period to be late, ie. weight loss, stress, illness.
If I were you, I wouldn't worry about it too much though, you are young and your body is going through many changes. If you start losing weight too rapidly, please go see a doctor. How do you feel in general? If you generally feel pretty healthy, but you're just stressing out about a late period, then try to relax and go about with your every day routine. Your period will come eventually.