Give more detail. Are you overweight?? Have some other health issue in family??
Nobody can tell you except a doctor, after doing a transvag ultrasound, or any number of other tests. It could be cysts, it could be a hormone imbalance, it could be a freakin brain tumor. (Not likely, but truly, a possibility). If you're desperate to solve the mystery, spend the money and get to a gyno.
If you have time to try other options, I recommend Vitex. It's a natural herbal supplement that helps balance your hormones, so if your problem is relatively minor, this can straighten it out.
I'd missed 3 periods in a row when I started taking it, then about a month after, I got a small period, then 2 weeks after that, I'm pretty sure I ovulated normally. We'll see in another week if I get a period (or a positive pregnancy test).
I take two 400mg capsules every day, one IMMEDIATELY after I wake up, before I shower or anything, and the other approximately 30 mins before I eat dinner. It's best to take them on an empty stomach so it absorbs into your system better.
Good luck!