What Causes Delayed Period While HPT Is Negative?

hi i have unprotected sex with my husband on 19 june,then i use i pill on that day on that month i have periods on 23 june now today is 2 sep. i does not have my periods.i also take the pregnancy test it is negative. i consult doctor she gave me tablets for my early periods but there is no result i does not get my periods.what should i do?