What Causes The Periods Comes Late

My girlfriend and I had unprotected sex on 16 may 2014. She took postinor 2 8 hours later. She had withdrawal bleeding a week later and have not had her period since. She last had her period on 5th may. We took a home pregnancy test on 19 june but it was negative. What could be causing the delayed period? She has a 5 week menstrual cycle and has Polycystic ovaries syndrome. Thank you