I get the pain also. It usually lasts a couple days after I play. Its always on my left side. I am a right handed golfer. I know itss the muscles so I dont worrie about it. Advil before the round usually helps it. I think if you dont get to play or swing alot then the muscles dont get used. When you do use them they get sore. Your Drs have ruled out internal probs so I would just pop some advil and PLAY GOLF.