What Causes Severe Pain In The Coccyx?

I have severe pain what seems like might be my tailbone. It is excruciating to sit! I did not injury my tailbone and in fact felt it for the first time a week ago after getting up from my chair, and it has progressively gotten worse. I have made an appt with a Dr. for tomorrow and suspect that I should get an MRI or CT Scan (which do you suggest?) I know it is not a soft tissue injury as I can press on the area without pain or tenderness. What types of reasons can be the cause? What tests should I ask for?