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how long has this been going on? have you had a chest xray or chest CT done?
It's possible that the dislocated rib never healed completely. This would bother you during certain movement or activities, such as rolling over in bed. It might even trigger muscle spasms in the chest wall or arm if the pain is bad enough.
I would suggest you get a CT scan of the chest. This would provide a lot more information than a simple CXR about your lower neck, shoulder, and chest. It should also show the status of the dislocated rib.
It would also help to rule out other possible causes of your pain.