The other day, I had a brief moment (a few secs) of blackout when I was standing in the bookstore. I quickly sat down to recompose myself. Within a few seconds, I was back to normal again. Sometimes when I squat down and stand up too fast, I will experience a brief moment (a few secs) of blackout. I don't feel sick or anything, just that brief moment of blackout. The doctor said there's nothing wrong with me. Has anyone expeience this?