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!
Give more detail. Are you overweight?? Have some other health issue in family??