What Is The Cause And Treatment For Irregular Menstruation?

hi, I am 25 years old And since May of 2014 I have had cases of irregular menstrual periods from May up until Novermber I saw my period twice and from November till date I have not seen my period . I am worried . I havnt not been sexually active since then and havnt taken any birth control pills . Kindly advice what steps I can take to solve this or prescribe any drugs I can take . I am also not pregnant.