Hi thanks for your question.
You had protected sex with your girl friend two days after her periods.She cannot get pregnant from this sexual act, as on one hand it was protected sex and on other hand it took place during her safe period.Reason for not having next cycle in time is simple.She had taken I Pill
which disturbed her hormonal
profile and she bled for 5 days from December 22 to December 27.So now you calculate her next due period from December 27 onwards.This is normal, menstrual cycle does get disturbed after emergency contraception
,there is nothing to worry.
Hope this answers your question.