Hi, Thanks for your query. Left side chest pain is what we are always afraid of as heart issues can give such symptom. However if the pain has appeared suddenly with shortness of breath and someradiation to the arm, pneumothorax is possible. This condition can appear with no reason. Sometimes sudden cough or sudden lift of heavy object can cause it. It is more possible if you smoke or have chronic lungs disease like asthma or bronchitis. If nothing like that is present, strained muscle is the most probable reason. Strained muscle usually appears only when the chest is moving, and should be completely gone when lying down and not breathing. If the pain is sharp and well localized, more intense when breathing, rib nerve issue is the most probable reason. In both cases, rest and anti-inflammatory medicines like Diclophenac should help. If the pneumothorax sounds possible, you should have chest x-ray done as soon as possible I do hope that you have found something helpful and I will be glad to answer any further query. Take care