What Causes Delayed Periods,lower Back Pain And Abdominal Pain?

my last period was on november 26...till now i didn t get my periods..i am living with my husband from january 9 th onwards till january 18 th we had intercourse..after that nothing has happend.and the thing is i have developed severe lower back pain since 2 weeks and since 1 week i have lower abdominal pain ,breast tenderness,mild head ache and some fullness in stomach,some time before i get my period i had experienced these type of symptoms,my periods usually irregular within 45 day cycle.i had done pregnancy test last week but it came negative..but the back pain and other symptoms noy relieving at the same time i am not getting my period.. im so worried please help me..