It could be a displaced coccyx...which w/b evident w/ an xray. If that is what this is....you're not going to like this, but....I had a displaced tailbone...and had 2 caudal steroid injections which helped tremendously w/ my sitting time...and ~ are you ready? ~ manual manipulation. Yep. But, it worked.
I have very little discomfort there anymore...and it was quite uncomfortable. The worst part was, every once in awhile, an extremely sharp pain would shoot into tailbone. That would only last a few seconds, but would catch me so off-guard that I wouldn't move for a few minutes after...or move extra careful.
Out2Lunch c/b right, too. You could have a problem w/ the lower discs. In that case, an MRI w/b best bet. But...a good start w/b the xray.
Have you spoken to a doc about this yet? The caudal injection was very painful for me...the first of two anyway. But...I believe that's bcuz I had problems for over 3 yrs by that point...so the inflammation had to be great. The 2nd injection went w/out a hitch and I don't regret it one bit. My doc that did the manips assured me that there was good movement on the coccyx and hopefully, that will be the end of my tailbone prob. No pun intended!
Good luck. Let us know how you make out.