我打棒球嘅...依家個膊頭每當揮臂就隱隱痛, 點算...

baseball commonly cause rotator cuff (the muscle and tendon on your shoulder) injury

before you see a doctor, you can try RICE:
R-rest
I- Ice therapy
C- compression, like using bandage to crepe it if it swelling
E- elevation, not to sleep on affected side, sleep on the other side with a lateral position

If these don't work, do see a doctor

see doctor http://www.leungbonesetter.hk/