People have reported that various medical procedures have been followed by coccyx pain:
When a procedure has been carried out in the pelvic region, it may be that pressure has been placed on the coccyx during surgery, or that the trauma of surgery has changed the muscular tension in the muscles in that area.
When the procedure has been carried out far away from the coccyx, it is more puzzling that people end up with coccyx pain. In some cases of gastic bypass, the pain may be caused by loss of a pad of fat protecting the coccyx from pressure, as Ed above suggested. However, Caroline Martin above found that in her case the pain started before she lost weight. And there are the other procedures that do not cause weight loss, but sometimes cause coccyx pain.
Dr Foye (see Doctors and specialists in the USA, New Jersey) has suggested that in some cases the pain might be caused by sitting for long periods in a reclining position in a hospital bed. This exerts a force on the sacrum/coccyx, a well-known phenomenon that is responsible for pressure ulcers, i.e. bedsores, at that exact location. This might cause not only the bedsores but also cause inflammation of the underlying coccygeal joints, ligaments, etc.