What Causes Delayed Period While HPT Is Negative?

Hi, I m married but we don t want children yet. I don t take any birth control pills but we r carefully with our sex n this happens on a regular basis but I m not getting my periods.my due date was on Feb 25 but still I haven t had periods, I m getting period pains n to confirm we also did home pregnancy test but it came negative. Wat shld I do..?