dear patient. there are different causes for the pain in the left lung region. i will highlight the most important causes 1.respiratory tract infection is a very highly probable cause. 2.pleuritis is second important cause. both of the above can be identified on a chest x ray.to help rule them out. 3.it can be a pain due to some external injury leading to intercostal neuralgia or costochondritis. it will go away within a week or so . can take analgesics to control it. 4.there is also possibility of a muscle strain if you do heavy exercises. 5.a rare condition possible called hiatus hernia causing repeated chest burns,acidity even if you take food or not. Hope i have answered your question.best of luck and take care