Thanks for posting your query.
Do you got some sort of chest injury?
, normal chest x ray
& ECG with history of swimming in cold
water suggest muscle sprain/ spasm.
Possibility of condylitis should be searched if pain is localized.
You should consult with internal medicine specialist and should go for thorough check up.
You should take strict bed rest
along with muscle relaxants.
You should also perform hot fomentation for getting fast relief.
Dr. Mayank Bhargava