It was from the blood rush to your head from standing up, like you have already pre-determined.
This has happened to me before once in my granny's house I was walking to the backroom for her sewing kit and at that time I was I think 13 or 14.
I remember standing up and just getting in the hallway. Prior to this we had been watching tv for like 2 hours. I stood up real fast and made my way to the backroom, and THUMP, blacked the forget out, fell over knocked over her bar stool and fell flat on the floor. Like 10 seconds later I got up and was like WHAT THE forget. Had a huge lump on the back of my head from that stuff dood
It felt just like when you make your face really red and the you start getting light headed and then let the oxygen back into your brain. Gives you a giant head rush and you occasionally see "stars" and then you have that 5% chance of just straight up passin the forget out.
I know this one kid who used to choke himself in school so he could get a headrush every 30 minutes or so. I remember telling him how stupid he was for doing it too. He died last year in a freak construction accident though, so that's pretty irrelevant.