What was this experience while sleeping?

A few years ago I had an experience I have never been able to understand. While I was sound asleep it felt like I was hit in the face with a softball of light. I felt my head literally whip back in the pillow from force. I immediately jerked awake and stumbled blindly to the bathroom, where it took a few minutes to be able to focus and actually see my reflection. It scared the bejeebies out of me. I've never experienced it again since, but would love to know if anyone had any insight. Thanks.