Well to tell you the truth, i have no idea what it is. It might be bursitis/impingement/tendonitis/subluxation. I seem to have symptoms of all of them, except the subluxation is a wierd one. I've never come across anyone else but me, but i can somewhat dislocate my shoulder voluntary. You can pretty clearly see the head of the humerus move away from the glenoid, and you can even feel the 'gap' that forms between the two. But that is all voluntary and causes me little-no pain.
My doctor officially diagnosed me with tendonitis via an MRI. He said i had minor inflammation and then said the single most idiotic statement that i have heard in my life, "You should probably stop working out." With that one statement, i lost all confidence in him.
I stopped working upper body for about a month, while i went to acupuncture/physical therapy (although the physical therapy was pretty half assed, but i didn't know as much back then as i do now).
I have been meaning to implement heavy Cuban Presses into my shoulder workouts, but i always forget to do them.