Sometimes it happens when a limb has had a shortage of circulation for a while, and then the circulation is restored. You might be in a position where you are resting on a limb in such a way that circulation is interrupted. As a child, I remember one night when I reached under the pillow and felt a strange hand under the pillow! I almost tossed my cookies. It only took a moment to realize that one hand had gone completely numb under the pillow, so my brain didn't get the message that the hand under the pillow was mine.
Emdrgreg