For 2 Months Now My Husband Has Been Having Sharp

For 2 months now My husband has been having sharp shooting pains in is tailbone. The pain is mostly constant but gets worse when he bends down. He has seen a chiropractor, who had xrays taken but they did not see anything that could be causing his pain. Also the adjustments didn't help much for the pain and seemed to make them worse. What could this be?