Please help me!!!! Shoulder pain

I recently hurt my shoulder at work while lifting a 70lb box.  I have a problem where my shoulder subluxes voluntarily and involuntarily, but the orthopedic surgeon (who specializes in spines) told me that according to the MRI i just have a small tear in a muscle. My shoulder feels very lose and my orthopedic surgeon just called me the next Harry Houdini (which tells me he really doesnt take my pain seriously at ALL). It has been 4 weeks since the injury and it doesnt seem to be getting any better. The original diagnosis was shoulder impinegment. When I place my arm behind my back it makes a popping sound and hurts, it is weak, and I cant hold anything out in front of me. But the pain doesn't feel like a sharp pain but more of a aching pain. I want to know if there are any shoulder problems that cannot be seen by the MRI? They did not inject any dye into my shoulder during the MRI. I dont know if it is tendonitis, shoulder impinegement, the subluxation, or maybe calcium forming between the muscle and the clavicle. What tests can be done and do I need to see an orthopedic surgeon who specializes in shoulders? My shoulder is not the same as it was, I am in alot of pain and I know something is wrong. PLEASE HELP ME!
Also, I have been in a sling for 2 weeks and then physical therapy for 2 weeks.

Thanks,
Brianna (21yr old, active duty military)