Shoulder is suspect
This problem sounds muskuloskeletal, which means it is either a joint (like shoulder), a tendon or bursae, or muscle or ligament.
From description sounds like it is stemming from the shoulder joint and one of the attachments (muscle, tendon..). It is the only joint in the body that has full rotation, so it is quite complex. Recommend rheumatologist (medical doc) or orthopedist (surgical doc).