I have been marathon training and increasing mileage gradually throughout the summer. On Sunday (3 days ago) my tailbone began to hurt when sitting. I had spent a significant amount of time in a car (4-5 hours) and attributed it to that. The pain lessened but after a car ride on Monday my tail bone hurt again when sitting and with other specific motions (bending over, etc.) and I could feel it slightly when running. I cross trained today and felt no pain with no pounding but it is still sore when sitting. It feels very much like tailbone pain and not lower back pain, so I'm wondering if this is the beginning of a stress reaction/fracture or something else.