What Causes Irregularities In Menstruation?

I'm going on to 8th grade. My height is 161cm and weigh about 133pounds. I started my period when i was 4th~5th grade. And my period is very irregular and i sometimes don't have it for 3 months but next time i have it for 3 weeks. and sometimes 7~8 weeks and than i don't do it for a long time. There's not that much blood just a little and sometimes it doesn't even come out.