What Is The Cause For Irregular Periods?

I am a 19 year old female. It's been six months since my period. I am not pregnant.The month the irregularity began i lost my virginity and was diagnosed with and ovarian cyst, now gone. I started my menstrual cycle late, 16 years old, andhave always been slightly irregular. What might be the reason my period hasn't come, and how dangerous could this be? oh and I have no symptoms while on my period or since the lack there of.