What Causes Delayed Periods?

I am delayed with my period about 5 days now and did pregnancy test its negative but I am still not confident to think it just about 2 week since had sex with my bf and we using condoms but maybe some came out not sure on that day but I am scared bec am so filter that day April 24..i should have period day 3 of of this month but none.Yesterday and today had been took 3 tabs Primulot Nor and had spotting now it does mean I stop my menstruation .