Why Are My Menses Delayed So Much?

hi, i am a 24 years old female. i was about to have my periods on 9th of this month but it haven't started yet. i have been having regular sex during this period without the use of any contraceptive. i did a pregnancy test on 17th and 23rd of this month and both the times it was negative. i generally have a delayed period (5-10 days delay). but this time its too late. i am confused, why am i not having my periods yet when i am not pregnant.
please answer...