I get the same thing. Your real body is probably not moving. It is more likely that you are just hallucinating the feeling of your body moving. Remember, when you start to enter a dream, your awareness will be in a new dream body. This transition can be weird, and often involves hallucinations of body distortions, movement, and other tactile wackiness.

You can try to induce your own tactile hallucinations, so you feel more in control and less surprised. When you know you are close, start imagining yourself floating, spinning, bouncing, or sinking. Whatever seems easiest. You could even involve this in an imagined dream scenario, like floating on ocean waves, or running down your street.