You probably experienced a short semi-lucid dream, where you began with the knowledge it was a dream, then immersed yourself into it and forgot about it. It helps if you write "This is a dream." on the back of your hand with some sort of writing utensil when your dream is hazy. I've also heard demanding lucidity works equally well, but that doesn't seem to work for me. I just wipe off my glasses, then rub my hands to gether and blink really hard. It seems to snap everything into focus for me.