Curious!

I got off of my birth control (Nexplanon) in July after having it for 2 1/2 years. I've had a regular period each month after except for this month. My period was scheduled to come on November 2 but it didn't. I've been cramping, experiencing really bad headaches and my husband says that I've been moody and irritable. I've taken several pregnancy test over the last week and they all come back saying Not Pregnant. Do you guys know what could be going on?