Hello, I am sure you'll get lots of good advice here. I'm not an expert, but others can chime in who are.
I'll start with the suggestion to go to a qualified Sports Med doc, not just your school trainer. I can't tell if you saw the doc yet, maybe you have.
Quick story, but in the process of recovering from a broken pitching thumb last spring, my son's (a 2013 P) ortho doc and a really really good pitching PT identified issues in his shoulder that concerned them. In hind site we think that this issue had probably been the cause of some 'tendinitis'. He got great advice, great PT, specific band work and other drills you might not be aware of, and it headed off what could have been an ongoing issue through HS and college bball. Maybe that is all you need.
Go to the experts, and be ready to learn all you can about how your arm works! Maybe you can head off a future problem.
Good luck - let us know how it goes.
You are just a sophomore and this is the time to really take care of your arm. Maybe you have something they can address and help you to build up for future endurance / arm health.