I've had episodes of left-side chest pain all my life (now 71), starting in my teens. At its worst, it feels like a fist inside squeezing some organ in my chest. But more often it's just a dull pain that makes me not want to lie on that side. I've had blood tests and been to cardio several times, always with a negative result which they are pretty confident about. I am fit, and carry no excess weight.
Over the years, it has dawned on me that these episodes are often associated with other people in the family having colds or similar. I don't often fall victim to such bugs, but I now suspect that the pains at least sometimes indicate that my body is being challenged by something. On other occasions, it's pretty clear that I am simply run down.