You "think" you are late--when in fact your early....In other words you cycle may just be longer than usual and you might have O'ed later than usual...OR...you happen to be one of those unfortunate women who doesn't get a + hpt until a blood test is done or not at all but still be pregnant...
I don't want to be a downer--but it is also possible to have an anovulatory cycle (where you didn't ovulate at all) and this can cause delays in menses..I only mention it because knowing is better than not knowing because it can and does happen with no outside medical help to resolve the issue--I myself have had a few of these before I figured out what sort of things to look for, etc. I had to take progesterone to get started and they never found a cause for my no-ovulation-cycle...I have also seen women have a cycle like this--ovulate when they should based on when they should have started their next cycle and get a positive hpt but not as far as long based on measurements etc, because they never had an af then got pregnant....I would def. check with your doctor if AF doesn't show and you keep getting BFN's...
Good Luck and Prayers!!!
Stephanie