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.