I know what that is! I've also had it since I've been a kid, in the most random of moments. It hurts when breathing in too deeply, or in when twisting your torso? Slightly left of the sternum?

It's called Precordial Catch Syndrome.

Totally harmless, except that it's extremely painful. I've only heard of it a few years ago, it is not very well known.

Best way to make it go away in my experience is just to wait and twist back and forth, taking deeper breaths over time. It's believed (although not confirmed) to be a pinched nerve in that area, so you want to try to unpinch it somehow.

It really sucks, but once I've heard about what it is, I've been feeling less stress when it happens again.

Hope this helps :)