What Causes A Delayed Menstrual Period With Negative HPT?

Hi,we r trying for baby since last 8months.I was getting my periods regularly with 28days but since last 3months its litle bit irregular.Last month it was delayed by 2days bfore that it was delayed by 7days.This month it is delayed by 5days . 2day i tested my 1st urine in pregnancy home kit but its showing negative........am i pregnant or not? pl help me .