Sometimes, the tail bone gets injured during delivery because there are a few pelvic muscles and ligaments attached to it and those get strained during a vaginal delivery. This depends on the mobility of that joint. Usually the pain goes away in a few weeks. However, if you still have pain, it is advisable for you to go to a good physiotherapist to get yourself treated. There could be a ligament strain/ muscle strain/ joint inflammation that is giving a long standing pain and needs intervention.