What Causes Missed Periods While On Nuvaring?

Hi, I'm only 18 years old and I have been on the nuvaring for about 2 years now. I was late changing it out one month but got it as soon as I could. While not having it I wasn't sexually active, but since then I still have not had my period. My period has never been regular, but it's been been about 2 months now and I know I'm not pregnant. Is there anything else it could be? Because it's really starting to scare me and I want to have other things to tell my mother when I go to her to tell her i've been late.