What Causes Irregular Periods?

Hi, I am a 18 year old female and I believe that there may be a problem with my menstrual cycle. I regually have periods which last for 21-30 days and then do not begin again for 1-2 months. I have never been sexually active and my period has been like this for a few years. I was just wondering if you have any advice and your opinion on of this could be PCOS. Thank you for your time