I Am Having Pain That Feels Like It Is In

I am having pain that feels like it is in my lung. It's located in the middle to upper part of my back on my left side. Was all across my back on Friday and at the base of my spine. Seems to be improving but slowly and still hurts enough to keep me home from work. I don't have cold/flu symptoms, no cough, the pain started on Friday after making two trips up three flights of outdoor stairs in freezing cold weather lugging bags of groceries. Just checked and my temperature is 101.2 on a digital thermometer. I have been taking Advil every six hours but it doesn't really help. Any idea what this is would be helpful. Thank you.