Hi Bobbyjean,
As you have mentioned, there may be many reasons to why the patient is experiencing the pain and stiffness.
Does your hospital have a clinical reasoning and planning sheet you can utilise? They help you to think about what you are doing and in most cases make it easier for you to understand whats going on.
From the subjective and physical examinations, list a few significant findings. Then make a few provisional diagnosis. Test these PDx. Find strategies that you will use as management/treatment. Reassess to test efficacy.
Things that you may need to consider:
diagnosis
management of inflammation
neurological deficit
ROM
neural tissue mobility
muscle length
movement patterns
stability issues
muscle activation issues
The above steps are what i would take when faced with a patient like yours.
Also, have a read of the discussion section of the attached article regarding management post minimal displacement fracture of humerus.
Hope this helps.
Cheers