What Causes Delay In Periods In Women?

I've had all of the classic signs that my period was gonna start. Normally I have my period exactly 28 days from when the previous one ends. I had cramps, bloating, fatigue on time, but that all stopped about three days after they started. I have not even had one drop of blood and it has been exactly 36 days today since my last cycle with bleeding.I also took a pregnancy test about a week ago even though I always insist on protection using both condoms and birth control. It was negative. I have not been stressed, been sick, or changed my diet. I'm going to go see my doctor. Any Ideas about what is causing this? (besides pregnancy)