I'm having this chest pain thats been bothering me for a few weeks now. It's mostly in the center but kinda leans more to the left side. It feels like a dull ache, quite like back pain, but its on the outer wall of the chest. I already had an ECG, and it came back normal, so my doctor thinks it might just be a muscle strain. But how can I really know for sure it's muscle strain and not something more serious?