Respected user , hi Thanks for using Healthcaremagic.com I have evaluated your query thoroughly . * There are various reasons as - Conditions of lungs ( infection , pneumonia , effusion , allergy , others ) - Problems of chest wall ( muscle stretch , sprain , blunt trauma , costo chondritis , rib fracture or inflammation ) - Heart related ( valvular disorder , cardiomyopathy , ischemia , infarction atherosclerosis ) - Pericardial conditions - Diaphragm problems * In short this needs extensive clinical work up with expert physician check up and series of necessary tests to find the cause . * DO NOT NEGLECT it and get an appointment with your PCP . Hope this will help you definitely . Welcome for further questions . Regards dear .