I agree, go see an Ob/Gyn. Have you had alot of stress in your life? Or alot of weight gain or weight loss? My SIL gained alot of weight after a severe illness and several operations and didn't get her periods for months. Go see you Ob/Gyn and have a chat to him/her about what te cause may be. Good luck.
You need to see an obgyn to find out why you are not menstruating on a regular basis.