Well, sometimes stress can mess up a cycle. I've had that happen before, so that's possible, and certain medicines can do the same.
Sometimes, though, you just have a "feeling", and if that's the case, CONGRATULATIONS!!!!!I never did take a test with my second and third. I just knew. I did with my fourth because I was just so shocked, after Dr. said we couldn't have any more.
Best of luck, and many prayers for you!