I've had similar experiences. If I'm laying on my back, I get this really intense side-to-side rocking sensations, like I'm laying in the bottom of a row boat being t-boned by waves. If I'm sitting up, I get a sensation similar to drunk-spins. Where you feel like your body is twisting or torquing while trying to sleep after a night of heavy drinking. I'm not sure exactly what causes it. My guess is that, because of powerful calm or focus, your body becomes disorientated and your inner-ear goes crazy. Like your body faking distress in an attempt to make you to right yourself, or at least assess your surroundings.

That's my guess anyway.