Are you like really active with a sport or something?
My friend has a similar problem. She's a hardcore dancer. I'm talking she dances almost everyday of the week and she's really active and she exercises a lot. Well, she will have a normal period then she won't have another one for like 6 months maybe (guess). Sometimes she has them for a few weeks in a row. Her mom is a nurse and she's been to the doctor and people have determined that when she has long breaks from dance (christmas, summer, etc) when she isn't active every day that it messes up her system.
Idk this might not apply to you at all but the only thing I can think of is something along those lines?
Gook Luck =)