I used to have a lot of shoulder pain from swimming, in the front of my shoulder on the biceps tendon. I found it was caused by keeping my arm straight through the catch and pull phase. This position has all of the work coming from the shoulder.

I changed to a slightly bent elbow, which puts the work on the lats. No more shoulder pain. It is hard to describe and easy to see. If you go on youtube and look for some videos on high elbow, early catch, you will be able to see what I am talking about. They call it open armpit swimming. Your coach should be able to help you with that arm position. It gave me immediate relief.

Good luck.