Given your description it's hard to say but you should consider a posterior lunate. I've seen a bunch and treated a bunch and they have been described in similar terms. If you follow the 3rd MC to its base and can reproduce your pain by pressing on the lunate while flexing/extending your wrist it's a good bet... Any good ortho/chiro/DO should be able to detect and remedy that. As you mention, it doesn't act like a typical lig. sprain though that's the first thing I would have wanted to rule out. Of course all the liability disclaimers apply - this should not be construed as advice, seek the help of a medical professional, blah, blah. Good luck!