Last night I tried WBTB and when I fell asleep in my dream all of a sudden I asked my self “am I dreaming?” After that everything was shaking like an earthquake and everything got blurred. After that it was a ND and I couldn’t remember that moment until a few hours after I woke up.
This “earthquake” is just vibrations. I always have the feeling while trying WBTB. First I fall asleep, then something strange in my dream happens (e.g. I was falling down and hit the ground). Then I get this strong vibrations, become aware, and see my room, quite blurred. And in the end I have an Out of Body Experience 
So the only difference is that You wasn’t aware. Maybe when you’re laying in bed try to think about going out of your body. It’ll help you to remember that you didn’t wake up, and this is still you Astral Body