Lung/back pain after running?

I run 20 or so miles per week and have recently noticed (with increasing intensity and regularity) a dull pain in my back upon inhalation. The pain is more prominent on my left flank, but also exists on the right side. The pain subsides when I lean to one side or the other, so I'm guessing the pain is coming from my muscles rather than my actual lung(s). My question is: What can I do to alleviate the pain without having to take an otc pain reliever before or after each run?