This happened to me really bad one night. I was at a party and I stood up from sitting on the floor. I don't remember what triggered it. All I know is that I woke up in the shower with my fiance trying to get me to calm down and holding me so I wouldn't hit him. I guess I got pretty violent. I don't know where it came from but it was scary as hell. Apparently I was like that for nearly two hours and I have no recollection of anything. That isn't the only time it's happened but it has been the most intense and longest I've had. Those don't come too often but when they do, it seems to come out of no where but I'm also usually really stressed around that time.