Sounds to me like impingement or bicep tendonitis..is it a deep throbbing ache? Bend your arm to 90 degrees as if you were in the middle of a biceps curl, rotate it in and out, if you feel up in about the middle of your side shoulder, there is a deep groove you will feel moving, that is your bicepital groove where the tendon attaches. Feel in this groove..is there where you are tender? Any pain that radiates down your arm? You might notice tenderness with internal rotation..oz is right, its sometimes difficult to determine ac joint impingement, biceps tendonitis, and supraspinatus problems.
I'd say biceps tendonitis, there is your internet dx lol....how if your range of motion in over the head activities? Any limitation in range or motion or pain? There are several PT tests for this, but typically biceps tendonitis presents with rotator cuff tears, and/or labral tears. It is a good idea to go see an ortho doc.