4 days late but negative pregnancy test

I normally have a 25 day cycle and am usually to the day. This month I'm four days late but have had negative pregnancy tests. I've tried the cheap supermarket tests, one of which had a really faded line and the expensive digital ones which came up as 'not pregnant'. Has anyone else experienced this? With my first two pregnancies I had positive tests on my first missed day. Not sure if I'm pregnant or not! I've been taking the Pregnacare before Conception supplement which I've heard can make your periods late. Any ideas/advice would be gratefully received!