What you're describing sounds a lot like the phenomenon frequently experienced known as hypnotic hallucinations or Hypnagogic hallucinations.
These are thought to be experienced as the body's natural attempt to stay alert, and also a wacky side effect from your brain being so active. Many lucid dreamers welcome these as they are often the hall mark of transitional phases, where one has the opportunity to pull off a clean WILD. Next time I simply suggest allowing yourself to relax and go with it.