Missed Period - NOT PREGNANT

Jessica • Married 5.7.16 --- Finishing my last year at OU and working as a freelance photographer.
Okay so I've been on birth control for almost 3 years. 
This has never happened:
I missed my period last month. 
This month I'm late, but not 6 days late. 
I have no symptoms, no spotting, nothing. 
Almost 2 months in a row now. Nothing. 
Took multiple pregnancy tests. Negative. 
Can anyone explain what's happening?