Esit: I originally said that your.muscles are creating waste chemicals quicker than your bloodstream can handle causing those bad chemicals build up locally. While this is true in a broader sense, I think it is more nuanced that that. Reading the article should give a better explaination
Yeah, I don't think this answers the question unless we're talking about the heart. The diaphragm is where all the muscle tissue involved in breathing is and that isn't what's hurting.