I missed my period and I never miss...please read!

Ka
So... I've now missed my period and have taken two tests, both of which were negative. Still having all my PMS symptoms. Mild cramps, and my boobs are killer sore. I've never missed a period in my life (even during college which was super stressful lol) and I'm usually pretty regular, like to the day regular. Little bit worried. What could this be?