I Have Not Had My Period Since January. Saw It

I have not had my period since January. Saw it last in December before 20th if I remember correctly. I have done pregnancy test 3 times 7 days in between but still negative. I'm confuse . My doctor ask me to take Primolut N 3 time a day for 10 days. That before the 10 days I would see blood spotting and menstruation will begging eventually. But he says if I see nothing then I should call him. Please this has never happened to me before. What should I do?