My 10 year old daughter is a competitive dancer. Today at dance she did a knee drop , where she goes from standing and falls to the ground, landing on one knee. Afterwards, her tailbone started hurting. It hurts to walk. When she changed out of her bodysuit, she noticed a red mark on her tailbone. She didn t hit her tailbone on anything, didn t fall on it and her tailbone never made any contact with the floor (or anything else). What could cause the soreness and redness?