I wouldn't call what your describing as "dreaming" in the traditional sense of the word. There's one of two explanations that I would bet explains what you're describing. One is that you are simply immersed in your imagination fully, your mind freed by meditation, the other Ithat you guessed), would be hypnagogia, which usually comes right before falling asleep but can occur in deep states of relaxation.

It's fairly easy to determine which you are experiencing. How does the "dream" start? If the dream is a direct extension of your waking thoughts then it's likely just a hyperactive imagination, the nature of these imaginings will be realistic logical scenarios with an obvious purpose and sense of urgency. If it blooms as if from nowhere, and is disjointed from your thoughts, it's likely hypnagogia. These dreams will eventually exhibit inexplicable situations and/or impossible imagery, the over-all feeling of the dreams are varied.