Need Help With Muscle Problems

I'm a 16 year old swimmer that swims for a club team and for about 5 months I have been having problems with my lower biceps and brachialis muscles. When I swim they get very tense, feel very tight and feel sore. Its preventing me from swimming and causing a lot of trouble. My coach thinks that it might have to do with tensing my arms up on my recovery but he is not really sure. I've been to a physical therapist and they also are not sure whats wrong, but he thinks it could have to do with rotator cuff weakness. My coach says that my stroke looks good which is why he is not sure whats wrong. I don't know if that is enough information to go off but im really just looking for anything, if anyone has had anything similar to this and fixed it or they know what was causing it. If you want to ask me any questions that could help, I will definitely respond. Edited to add diagram showing where I feel it