Hi precious,
In my teens, when I was experiencing the same problems as you, and was a average weight, my periods stopped for 3 years. I think it is just our bodies way of saying it cant cope with what we are doing so it hibernates to protect itself. If you are worried, go and see your doctor. My periods came back when I was about 18, and are still regular to this day, and I am 47 now! I have had every eating disorder under the sun, I think, but my body is strong, and I try to eat healthily whenever I can, although not doing that brilliantly at the moment. Good luck.