A phantom limb is the feeling that amputees often have that the missing limb is still attached and giving sensations. More than 50% of amputees experience a phantom limb. Phantom breast sensations can likewise occur after mastectomy.
Unfortunately, sensations from a phantom body part are usually painful and that pain is almost impossible to treat, even though the pain is very real, not imaginary.
There is a section of the cerebral cortex of the brain where nerve signals from major body parts go. In a typical drawing of this somatosensory cortex, the amount of cortex dedicated to a particular body part is represented by the relative size of a drawing of that body part. The face, lips, and tongue use a large part of the somatosensory cortex, and that corresponds to our experience that these parts of the body are well-innervated and particularly sensitive, compared, say, to the middle of the back.
There are two of these somatosensory cortexes, one on each side of the brain. The left somatosensory cortex represents the right side of the body, and vice-versa. Normally, if your right hand is stimulated, nerves would fire in the left somatosensory cortex, in the area corresponding to the right hand.