Why hasnt my period come yet?

Ive been having my period for about two years now. I never know exactly when its going to come though, some times im on my period for like four days then have another one in less then a month, then some times i have it for around seven or eight days and i dont get another one for about a month and half. I am a virgin so theres no way Im pregnet but i havent had my period in about two months. help please!