Another thought: try moving your saddle a few cm forward on the rails.
I was initially getting big pain in my right arm on the new bike, thinking "shorter stem", "frame too big" etc. My right arm's effectively a bit shorter than my left because of a previous accident so It was stretching massively and that's why the pain ended up there, but that minor adjustment has totally sorted everything out.
Another thought: try moving your saddle a few cm forward on the rails.
I was initially getting big pain in my right arm on the new bike, thinking "shorter stem", "frame too big" etc. My right arm's effectively a bit shorter than my left because of a previous accident so It was stretching massively and that's why the pain ended up there, but that minor adjustment has totally sorted everything out.