You should see a sports medicine doctor/orthopedist. Consider MRI shoulder.
You did not specify which part of arm was in pain. Also consider MRI elbow.
When athletes throw repeatedly at high speed, the repetitive stresses can lead to a wide range of overuse injuries.
Common throwing injuries of the shoulder include SLAP tears, biceps tendinitis and tendon tears, rotator cuff tendinitis and tears, internal impingement, instability, glenohumeral internal rotation deficit, and scapular rotation dysfunction.
Common throwing injuries of the elbow include flexor tendinitis, ulnar collateral ligament injury, valgus extension overload, olecranon stress fracture, and ulnar neuritis.
Hello~I would definitely see a chiropractor that specializes in sports medicine. It sounds like pulled or pinched muscles or nerves.