Something I didn't see anyone mention was the fact that you were bent over your bike (thinking you were gonna spew...) when you passed out. I am not good enough with science to explain this in detail for you but it is something we repeated many times in the Army while stretching after a good workout - keep your head above your heart. You had a lot of blood pumping at the time and you made too much go to your head - no big deal you will live.