Late Period. Negative Test.

Hey ladies. I've been TTC (trying to conceive) for the last two months, my periods came when they were supposed to. They last about 3-4 days and are regular. I'm on a 24 day cycle. My period was supposed to come on the 25th and hasn't came. I took two clear blue digital pregnancy tests and they have both came out negative. Why? Has this happened to anyone else?? I know it can't be that I'm stressing bc my period would usually only be heavy if that was the case. And my period is only late if I'm pregnant. That's usually the case. I don't know, I'm confused. Help pls.