Two weeks late + four negative tests = going slightly mad!!

I have had a dead on, 28 day cycle since I was about 16, so it is unusual for me to be two days late, let alone two weeks! Took my fourth test this morning and it was a clear negative. When I got pregnant with my first, I fell straight away (literally a first time's the charm situation), but it took almost a month for that BFP to show up! If I'm not pregnant, how long do I leave it to contact the doctor?