What Causes Delay In Periods?

i had sex on 20th november ,night,and had an unwanted 72,the next morning...then i had bleeding on 29th november,for around 4 days....and had tenderness of my breast...i am supposed to have normal periods,around 11th december,but it has still not happened.morever,i had a beta hcg test done on 30 november,it came negative....by when am i supposed to have my periods tentatively