What can cause bad shoulder pain and painful subluxation but normal mri?

Shoulder subluxation. Mri not allways the answer. Sometimes an arthrogram has to be performed to reveal tears. Also a good clinical exam by orthopaedic surgeon is very helpful.
Stretched capsule. Sometimes you need an arthrogram, injection of dye into the shoulder, before the MRI to make the diagnosis of a stretched capsule. With your description and a positive clinical examination, you might want to have an examination under anesthesia and then an arthroscopic repair. See an upper extremity surgeon.