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.