I'm dealing with this right now, and it isn't fun. It is not, however, carpal tunnel, or even the beginnings of it. When the left hand is bent over and fingers curled in with force like you must do to play in the upright position, it is not natural and puts great pressure on the bones in the wrist, which are just floating around in there. The wrist muscles typically are not developed, particularly on the back side, and it is those muscles that hold the bones in place. When those bones shift out of place, they interfere with the tendons that run out to the thumb and fingers...so what you are experiencing, and me too, is actually tendonitis. The bones can be pushed back into place, but you'll have to do exercises (so my therapist tells me, and it makes sense) to strengthen the muscles to keep those bones in place. So, I'm taking a break for awhile to let it all heal up and strengthen those muscles. In the worst case scenario, I'm going to look into a reverse 'harp that I can play without crossing my hands, if I can find one.