It is difficult to rule out any type of medical problem. Having said that, many hand problems can be attributed to something fundamental in the seated position. Even focusing on on a good seated position assumes good posture and freedom from excess tension. It has been my experience that many students, and players carry excess tension in the shoulders. I will try to explain an exercise I do with my students to rid of excess shoulder tension. Perhaps a friend could assist you in trying something similar.
First, get in a good seated position with good posture. Rotate the pelvic bones thus instantly increasing your height and helping to promote good posture. Glance at an imaginary horizon off in the distance with your arms fully raised. Maintain that eye contact as you bend over allowing your head to move towards the floor. Maintain the eye contact on the horizon helping your self to lengthen and widen (The opposite would be to look down and tense up the back muscles as you bend over.) You want to create as much distance as possible with the head and pelvic bones. As you complete the movement, allow your head to look at the floor. To return to the seated position, look at the horizon, raise your arms and return. The exercise should help to perfect a good seated position with good posture.
Next comes the part where you will need assistance. Allow your arms to relax as much as possible, generally hanging towards the floor. Have your friend get a good grasp of your arm somewhere above the elbow. Your friend needs to simply lift your arm. Your job is to not help at all. Your job is to "play dead" having your friend do 100% of the work. You very well may be surprised that this is far more difficult that it seems. Congratulations, you have just discovered counter-productive tension. To get a better sense, you can trade roles with your friend thus getting a better sense of the tension. Play around with that for a while. Make it a quick daily routine. The good news is, a little awareness goes a long way. You should quickly feel more comfortable and that will show itself in your playing.
On the seated position, it is very easy to gradually evolve into something less than optimal. Fortunately for me, my own students keep me thinking about it and that keeps me on my toes. You should be able to maintain that very good posture while keeping the thumb behind the neck and fretting the 19th fret of the 1st string with your 4th finger. If you aren't able to do this, the position of the guitar needs to be adjusted.
Experience has taught me that the things I have written above are just as important as any examination of the technique and mechanics of the hands. In some ways, I feel they are actually MORE important. Having good posture and good physical poise often promotes the hands in finding easier ways to do things.
Keep me posted. I'll reply as often as I can. Oh, and as always, Happy Practicing!
Dr. Todd Tipton, classical guitarist
Cincinnati, OH, USA