Offseason workout

I am an american pitcher who lives in the Netherlands. Baseball here almost year round- with games going from March-June and then from August-October. Then ‘spring training’ starts in November, and goes on until the start of the season.
I throw a fastball (around 65) and a knuckle-curve, both with an arm angle of around 1:30. My dad and my pitching coach agree with me that I do use my whole body when I pitch and don’t simply muscle the ball to the plate. However, I’ve been experiencing pain behind my shoulder and in my upper arm muscles during and after throwing for the last couple of years. I’m wondering what I should do this offseason to improve my pitching, but most importantly get rid of the pain. My coach is willing to work with me on this. Does anyone have any suggestions?