What possibilities could delay your af?

So I am a week late. I've taken 4 hpt and they were all negative. My af is like clock work, I get it on the 14th every month and it still hasn't showed or have I had any af like symptoms. (which I usually have a week before she shows) I know I've been stressed lately but that usually doesn't affect af, usually makes cramps worse lol. Anyone else have any other ideas? I had my hopes up for a bfp since it was this late, but nothing..