Cody's explanations sound generally good to me, except that occlusive ischaemia would have caused you to black out on the mat (it only takes a few seconds). Diego's vascular spasm is also a possibility, but I'd be banking on vagal inhibition of the heart (as MST says, when your HR drops your BP follows). With your existing low BP, you would be more vulnerable to such an effect...

...especially at the stroke of midnight on the winter solstice!