I think you're referring to sleep paralysis? It's our bodies natural defense mechanism against us acting out our dreams. For instance, if you dream you're walking across a forest, but you live in a 17th floor apartment, that could end very badly, unless your body takes steps to paralyze us while we dream. It sounds like some wires have gotten crossed! I think it's not unusual for this to happen occasionally, but if this has happened several times recently, you might want to get it checked out.