What Causes Delayed Period While UPT Is Negative?

Hi I am over a week late on my period, iv took several pregnancy tests which are negative, iv experienced diarrhoea and a few stomach pains the last week, suddenly I got sharp pains in the left lower side of my stomach and I keep feeling like I need to poo, Any pressure make the pain worse on the left side other than that its settled but still uncomfortable, my last period I experienced clots after id come of so iv been to the docters and had swabs, they also took a pregnancy test which was negative but said I could of been caught after which wouldn t of shown up yet, also by having swabs I could miss carrt if I was pregnant, iv also had a ectopic before which worries me.