What Causes A Delayed Period Despite A Negative Pregnancy Test?

Hi, may I answer your health queries right now ? Please type your query here... I missed my period in this month.its due by around 13 days.I have had pregnancy tests 4 times at home and the results were negative.two months back I faced the same problem and doc adviced me to have deviry 10 mg for five days,one tablet a day.last time my periods started after the first tablet itself but this time I have had 4 tablets still no periods.Kindly advice and why is my period getting delayed.last month it was on time.