What Causes Missed Period And Negative UPT While On Birth Control?

hi i started birth control aug. 3, 2010 my last period was july 24, 2010 i missed my period for aug i took three pregnancie tests all came back neg. i now just ended my 2nd pack of pills and have missed my period again. i havenever missed a period before and i have been getting some cramps but no bleeding. im not sure if im pregnant or not