What Causes Late Periods After Taking Emergency Contraceptive Pill?

Hello doctors... please help me out...
i had sex with my gf in dec 21st 2013...
n her last periods was in 9th of dec...
during sex the condom was leaked.. n i was stressd weather the sperm goes in to her vagina or not...
so my gf take unwanted 72 in 24hrs... thats 22nd of dec...
and after taking unwanted 72....
6 days later that is 27 around .. she got a flow for 1day.. according to her its not a normal periods...
after that till date she didnt get her periods.....
as we already test urinal pregnency test.. all results are negative...
but still i m in depressed why she didnt get her periods till date....
plz help me out doct....