What Causes A Delayed Period With Negative Pregnancy Test?

I had sex in dec and got down in dec end as well. Since jan I have not been down and taken a pregnancy test which cm negative. It's been 4 days that I had completed the course of meprate for 3 days with 3 tablets each day. When should I get my periods ? Is there a requirement for me to take pregnancy test again ? Also, if I get my periods in a few days then can I be sure that I m not pregnant.