I've had this happen a few times over the past three years. It is very scary though because there really is no/not much warning of this happening. My most recent one was a month ago yesterday. I remember walking in my bathroom, I got really bad vertigo out of nowhere and just remember grabbing for the towel rack. I ended up taking down the towel with me as I hit the floor and hit the back of my head again. I don't know how long I was out for, but when I came back around, I had my four year old sitting on top of me with the phone saying, "Mom you hit your head again. Are you okay?" Of course I ended up at the ER to get checked out, and had another concussion with a sprained neck. PCS can be so agravating sometimes!