Did you experience it in slow motion? Years ago, I was walking up a steep flight of stairs in a house that was built in 1865 and my (then) husband and I were restoring it. In the 1800's, nothing was true to size or form and the staircase was winding, too. As I got to the top, I reached for a stupid piece of lint that was on the hall runner and I somehow pitched backwards and went down the stairs. It all happened in slow motion and it seemed to keep me from getting injured. I know that sounds weird but it's true. You must have just gone "thump!"