as you know you cannot be diagnosed blindly over the internet. you need to see a good orthopedist (find one who has an expertise in shoulders) and yes you probably will need an mri.
i play softball and i stretch before every game i throw righty and have pain from my shoulder down to elbow what is this. i had a shoulder injury that was never checked on about 6 years ago i jammed it on the ground. would an mri be a good option?