Period two weeks early

Ladies: this morning I passed blood clots, identical the those when my period begins. I'm two weeks early - by the Glow calculation, in fact, this should be my fertility window. I am now realizing that I had PMS-like symptoms yesterday (backache and others but mine are usually minor so they didn't catch my attention). I can say with confidence that I am NOT pregnant, so this is not any sort of implantation bleed. I'm usually very regular.