Hi, it does need more clustered tests to determine what structure more likely involved. But from the info you have provided, especially " arm across body painful" is a typical AC joint sprain symptom. Try to pull your left collar bone downwards and backwards and then lift your arm up, see how you feel? if it feels better, most likely is your AC joint. depends on the severity, you might need more time to rest and rehab before you go back to play though. taping also helps.