What could it be? It's almost an acute pain, and whenever I run hard, it affects me (I would describe it as having the wind knocked out of me very slowly and increasingly). If I go for just a solid pace run, it is uncomfortable but I can run through it generally. On easy runs, it does not affect me at all and is hardly even noticeable.
It has affected me very sporadically for the last year, but this last month it has been a serious problem. Maybe a pulled muscle underneath the rib cage? Small crack in the rib? I will probably need to get an x-ray in the next week. Anyone else had this pain?