It was like I was falling down a hole, things went dark around the edges, then the dark contracted until I was unconscious. My friends freaked out and took me to the emergency room since it was only a week or two after I'd been knocked out by a horse at a show (struck in the face, out like a light). They thought it was a delayed effect from the concussion.

We never did figure out why I passed out. When we got to the hospital I was coming back around but hyperventilating, so the doctor attributed it to that. I was NOT hyperventilating before I passed out, though.