ANDY'S ANSWER: The thing about your menstrual cycle during puberty is that is often are often irregular. Sometimes it takes years to settle into a predictable cycle, and some girls are never regular.
Even for girls who have somewhat regular periods, other things can affect them like stress, illness, medication or exercise. Dieting can also affect your period. If you are underweight, you may have more issues with missing periods than someone who is not.
The best thing you can do is try an have a balanced routine of healthy food and exercise and get adequate sleep which is important for everyone, but especially for teens whose bodies are still developing. Your periods should eventually even out and get more regular as time goes on.