It is most likely due to thoracic outlet syndrome, an impingement of the brachial plexus that results in numbness and pain in the shoulder that usually radiates down the arm.
Usually caused by muscle imbalances & flexibility loss in the shoulder or shoulder girdle. You need a qualified assessment and soft tissue work, stretching and corrective exercise and program design to eliminate the imbalances I.E reduction of pressing movemts and lots of retraction and external rotation
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