Upper left side chest pain, should I be worried? What’s the usual rea?

Is it “pain” or “muscle discomfort”?

I had breast cancer as a man and I get chest discomfort due to scar tissue, but its muscle related per my oncology team and sought a PT.
Suggest reaching out to your PCP for a checkup. Do not delay.

You’re scared of what you don’t know. Better to know more than less. See a cardiologist. She’ll give you the news - good or bad. But at least you’d know what course to follow.