What Causes Severe Pain In Tail Bone?

I am a 25 yr old male and for the perhaps the past five-seven years I have been experiencing a sharp excruciating (and slightly embarrassing) pain on the right side of my tail-bone maybe 2-3 inches above the rectum area which pretty much only occurs after orgasm. The pain isn't instantaneous; I can feel the onset and it usually peaks after a couple minutes and will last for 10-30 minutes. However, sometimes I feel if I can relax and lay down it will subside. Since the first occurrence the pain happened maybe once every six months. Lately it seems to happen more frequently almost once a month or more. I can remember only one time when the pain occurred while sitting, I was in a car which hit a very small bump. I am physically active, but I sit in a chair at work for perhaps 8-12 hours a day. Any ideas what this could be?