You've had the tests. It's anxiety. After the back the shoulder is the most popular muscle group to be subject to chronic stress. Plus you are left handed, and that is the side you would react to in the "fight or flight" reflex, clinching and shrinking and rolling the shoulder over, all done unconsciously.
As far as a rotator cuff, forget about it. It's a catch all, unless you are a major league pitcher or an NFL quarterback.
Why doesn't it hurt during sex? Easy, your thoughts are on another muscle.
You're at the age that if you understand you have an anxiety illness you can get help with it and learn how to deal with it; otherwise, your symptoms will get worse, and you will go from doc to doc, and end up a true mental and physical basket case. Do something about your anxiety now!