I had my last period march 6 2012 and now it is two weeks late and I have been having unprotected sex since my period went of I have taken three pregnancy test with negative results there is still no sign of my period I have gotten nervous and I would like to know what is the cause of this

It may be just an off cycle and you will skip this month or it will be late. You could either see your doctor now about having a blood test done for pregnancy, or wait until next month and see if your period shows. If not, then test again at that time.