anything I post is just friendly suggestions, not professional advice. I also would like to know what stroke you have pain with.
anatomic position is palms forward, so your arms are supinated and the radius and ulna are not crossed. can you describe where the pain is in that position using medial as towards your body, lateral as away from it, and anteriorly and posteriorly.
also, is it worse when you first wake up, or after you move around for a while?
before you swim, early in the swim, or after the swim?
then put your hand on the wall at shoulder height with arm bent and push, does that hurt. does pulling hurt?
now grab something immobile and try twisting clockwise and counterclockwise. do either of those hurt? (need to know which arm we're talking about.)
I'll make a guess if you answer some of those questions, or maybe someone else will know too.