Have you counted the days between? The way you describe it sounds like you have it every other month. If that's the case and it's relatively the same number of days between it shouldn't be anything to worry about. I personally have 26 day cycles. Some people have 28 day cycles and some have longer. You might just be lucky to have much longer cycles for now.
Mal named everything else that would account for an irregular cycle.
If you'd prefer to have yours on a monthly basis you could easily get birth control pills. If you are sexually active and not ready for kids then this is a good idea anyway. Plus it keeps us going back for those wonderful
yearly exams. My mother was extremely irregular and her periods were long, heavy, and painful. She went on the pill and it shortened her periods, made them less heavy, and helped with the cramping. An irregular period is quite common, just annoying, inconvenient and draining on a girl usually.