What Causes Missed Period After Taking Norethisterone?

Hi. i last got my periods on december 14th but used norethisterone to stop the periods the following day. In january 2014when i felt my period was due i took norethisterone to delay the periods and somehow i did not receive as well as upto date feb 17th i havent received. Tested for pregnancy but just 1 line and a second line so faint. waited for three four days today and still same result usual strong line and very very faint line. as i have all the signs of getting periods my doctor prescribed primolut n 2 tabs today and tomorrow as he thinks its just dirt stuck there. Please advise.