I had a similar experience once late at night watching TV. Mind numbing task I know, but, i was sitting on the couch with my legs up and body upright, then I saw my body sliding down but "I" was still upright wathcing TV!! it lasted only a few moments because my mind finally relaised what was going on and I felt like I was 'snapped' back into my body. Like a similar effect you'd get from flinging a rubber band.
Bloody freaky isn't it?
-QM.

