What Causes Missed Periods If Not Pregnant?

i am 20 years old and am in the navy and married to another sailor. I have not taken birth control since early sept. 2010 and i haven't had cycle since Nov 29,2010 and it's now Mar 5,2011 and still no change. i have taken many hpt's and most of them came back negative except for 2 or 3 which had faint lines on them. if my tests still come back negative then what could be possible answers for me?