I've had pain on the left side of my rib cage for more than two weeks now. I'm pretty sure that I did something to my serratus antirior, the muscles that come from under the lats and connect to your external obliques, they help hold your rib cage together. Don't do anything that will tax your chest area muscles for a few days and if it feels better, then your know you probablytweaked a muscle. If not, go see a doctor. Mine is been hurting so long because I keep aggravating it with exercise.