Displaced tail bone
The coccyx is made up of three vertebra that sit at the bottom of the spine. For most people they are totally insignificant but these three bones are generally fused together and rarely move unless one has an unfortunate accident such as slipping downstairs or slipping on ice.
When this happens there is generally a sharp pain followed by a persistent pain of varying intensity around the base of the spine. When damaging the coccyx due to a fall, most often the coccyx is pushed forwards and it's a simple but uncomfortable operation to pull it back into place.
The coccyx can also be displaced during childbirth where it is more typically displaced backwards or to the side. The pain may not be initially felt during the delivery, but afterwards as the body begins returning to its pre-pregnant status. Treatment again is usually a short sharp readjustment.
A typical treatment is for a licenced therapist/ doctor to physically insert a finger into the anus and reposition the coccyx under the influence of some local anaesthetic.
This video has some good advice that can be incorporated into the massage if no qualified treatment is available. The methodology here is from applied kinesiology.