I suspect you are right in the RSI type scenario but it aint necessarily so!

All instruments stress parts of the body differently. I changed Accorion in June and still have an elbow injury, the rib cage and shoulder have cleared up.

I recall Luke Daniels (Broderick and Riverdance new york) I beleive had RSI and spent a long time playing with his button accordion bolted in to a frame, this took the weight off him. It is bound to be more complex than that.

He surrvived and is still playing but yes - take the instrument to the Doctor and Demonstrate. I did this with a fiddle injury once when I was playing alot and had neck pain, a simple adjustment of the shoulder rest - spotted by the physio as a non musician - cured it almost instantly.

Good luck

Andy