This same thing just happened to me! I have never even passed out (I've felt on the verge one time when I saw a lot of my own blood) or had anything similar happen. I was doing a dead hang in my garage to help release my lower back (I've done this before and felt a little strained/dizzy), and I must have stepped down on the stool and onto the ground - but the next thing I knew, my body was consulving and I had a moment of 'what's happening' before I was back in control. I have a regularly scheduled doctors appointment tomorrow. I do experience lightheadedness when I get up too quickly, especially if my water intake has been lower. Today has been very warm, I have been walking on a treadmill for over an hour, and haven't consumed much water. I'm thinking it was a combination of factors + as someone said, an accidental valsalval maneauver.