What Causes A Delayed Period?

I am a 23 year old female. I had the Paragard IUD inserted on November 13, 2012, three days after I had finished my last menstrual cycle. That menstrual cycle, which began on November 5, 2012, started two days after I stopped taking Micronor (and that period was 2 weeks late). I am now over a week late with my period, but have not had sex in over 2.5 months so I know I am not pregnant. Any ideas? Could this be due to the discontinuation of the Micronor? I have an appointment with Planned Parenthood on Friday, December, 14, but am growing anxious. Additionally, I am a graduate student and last week was finals week so I have been considerably stressed out, if that could potentially contribute.