Whay a coincidence! My left arm's going through just that, and playing the melodeon is giving me hell. (Makes a change from me giving everyone hell on the melodeon...)
One of our side is a nurse and has suggested tendonitis. I don't know. There's lumps and bumps in the upper arm muscle, and the pain seems to centre about halfway up, in the curve of it. Then when I drive, or play my box, it goes into the muscle on the outer edge of the elbow, and then shoot down the outer edge of my forearm. I slather diclofenac on my arm before I play, and wear an elbow support. It hasn't been helped by my buying a bigger car which means I have to reach further for the gear stick.

I really must go to the doctor!

Andrea