May I suggest a very normal human experience as the explanation?

What you describe is a very typical example of what is called a hypnogogic illusion. It is caused by an unusual blending of sleep and awake brain states. You straddled the line, for a moment, and your awake brain didn't understand.

Here's a link:
http://watarts.uwaterloo.ca/~acheyne/S_P2.html

Best,
Lar