ANDY'S ANSWER: Most teen's periods are very irregular, sometimes missing whole months or coming earlier or later than expected. They should slowly start to regulate as you get older, but even so, very few people are so regular that they can count on their periods arriving every 28 days without fail.
Other things can affect a period as well, such as stress, illness, etc. Of course the big one is pregnancy. If you've had sex, that is always a possibility even if you've used birth control.
Your body will give you signs that will let you know a period is close and you will learn to recognize these signs which will help you be prepared. Things like an uncomfortable feeling in your abdomen, bloating, hungrier than usual, weight gain, etc. Everyone is different but you will learn to listen to what signs your body gives you.
Below is the recording of Andy answering this question.