What Causes Missed Periods, Back Pain And Constipation?

HI my name is Temika Moore I'm 31 years old I started my cycle on Jan 9th 2014 my cycle come between 21 to 24 days. Today is Feb 11 and still no signs of my period. I took two pregnancy test and they are negative. I am very bloated constipated and tired most of the time. No nausea are vomitting. I have severe back pain at night when I sleep. I have never missed a period can you tell me what may be going on please?