What Causes Delay In Menstruation Other Than Pregnancy?

gud evening doctor, i supposed to have my menstrual period last december 20-23,2013 it's my target days either of those days. i had sex with my husband dec.17,2013 onwards,i took also ovamit as my doctor advised last september 2013. could i be possible pregnant? i tried to have some test of pregnancy test both blood and urine but it turns out negative result, however i researched through internet that it could be false negative specialy if hcg level produces low..december target date, january 2014 and until this february 2014 stil no signs of menstrual period but i am experiencing symptoms of pregnancy,but it's negative..pls.help me..