What Causes Delayed Period While HPT Is Negative?

I am 23 years old married women.
I was having regular periods but from the month of December everything changed.
I got my last periods on 16th Dec, 2013 since then i have not got period till today 7th Feb, 2014.
I have checked with the physician she gave me Premont-n tablet to be consumed for 5 days twice in a dy.
But it didnot helped.
I am not pregnant done with pregnancy test thrice.
Please help i am worried.