The pain you describe is probably muscular. You are right to have stopped the heavy lifting to rest the involved muscles. The dark sputum is unrelated to the muscle pain but almost certainly an indicator of chronic bronchitis, due to your long-time smoking of cannabis. That you can run 3 to 8 miles makes it almost certain that you have not yet developed emphysema from smoking. Nevertheless, it would be unwise to assume that your symptoms are muscular. With your smoking history, you should definitely have a chest x-ray done.
I am not a Dr. but I was recently diagnosed as having pulled or damaged the intercostal muscles between the ribs on my right side. Pain is definitely increased if I pull my shoulders back because the rib cage expands and thus the intercostal muscles do too. Mine came from three weeks of coughing after sinusitis.
However, that would not explain the dark phlegm. You didn't say whether you mentioned this symptom to your Dr. Did you? And did he still think it was muscular?
If it continues, you may want to revisit the Dr. and reveal your smoking history and see if he feels further evaluation is warranted.
Thanks bmw1, appreciate your response.
I have over the summer been working delivering water which was very heavy work, lifting down 18.5 litre bottles from my truck always taking the load on my right shoulder then carrying them two at a time all over the place! The first time that I started to get these chest pains was strangely when I was on holiday and was not lifting anything heavy. I am no longer moving water about my chest pains seemed to start as my heavy work stopped (give or take a few weeks)
I have seen my local Doctor about this problem, I did omit the fact that I smoke cannabis as I cant have that on my records, and didn