As
@berkeman pointed out, this is a very tough problem.
Diagnosis starts with a concept: range of motion. It is used to figure out if there is any impairment/problem with:
muscle tissues,
connective tissues,
bone,
or a mixture of the above.
If you can move your hurt arm (even with some pain) in all of the ways you can move your good arm , the problem not a major concern.*
If you cannot do this because of severe pain or your hurt arm no longer can "go" there, see a physician.
If the pain does not slowly go away, see a physician.
The next big complication is referred pain. It hurts in a place on your body where nothing is wrong at all.
Example:
Certain shoulder injuries (e.g., rotator cuff ) often hurt in the middle of upper arm, not the shoulder where the damage is. Rather, it seems to you to come from the insertion point of the deltoid muscle. The deltoid tuberosity.
I am in no way suggesting this is your problem. An example only of referred pain. You can have shoulder pain during a heart attack, just to be clear.
PF cannot do any kind of medical diagnosis. So get help, do not self-diagnose.
* this is a paraphrase from my notes I took that came from a textbook:
'Differential Diagnosis for Physical Therapists Screening for Referral' C. C. Goodman, T. E. K. Snyder 5th Ed. Older edition.