There could be many reasons for missing periods. Do you work out heavily, stress, etc. You might want to check out some books, Women's Bodies, Women's Wisdom by Christiane Northrup, M.D. It is supposed to be very complete, if you don't want to buy it, check the library.
I don't like running to the doctor, but in this case you might have to get it medically checked out, so you know what you are dealing with and go from there.