trecylwv,
There are some people who have accessory muscles in their hands. It is not common, but the extensor digitorum brevis manus muscle is often misdiagnosed as a ganglion cyst.
If it is thickened tissue, that is not normal. It is usually due to inflammation. But, what is causing the inflammation?
Even though you have labs within normal limits, that does not rule out all of the arthropathies. It may be that you are just presenting at a very early stage of a disorder, before the "classic" symptoms have appeared.
About all you can do at this time, is treat your symptoms, and keep seeing the doctor every so often. If you do not say anything, the doctors are going to assume you are doing okay. You need to keep telling them that you are not doing okay, and that you need some answers.
It may take some effort and time, but be persistent. Sometimes is just takes one doctor to have an "ah-ha" moment or for enough symptoms to show up, to finally get a firm diagnosis.
Good luck.