Many young girls find they have irregular periods up until their twenties when this schedule smooths out. If you're having sex, it could throw off your period.
Keep track for a few months and see if you can determine a pattern. In addition, talk to your mom and see if she had the same problem when she was young. It could be a one time anomaly and won't happen again, but if this were me, I would start keeping track.
Two pieces of advice - always, always have something with you, so you're not caught off guard. Two, always, always use birth control. Every single time. Don't rely on your partner - you need to take care of you.