The good news is that (other than the pain) it is completely harmless.
The way it always felt to me is like someone wrapped a small wire around something tender in my chest, and if I tried to breathe or straighten my posture, they would yank on it. I’d get it anywhere from a couple times a week to once a month. Then one day in my mid-30s it just stopped.
From what I understand this is relatively common. I was so grateful for the person on reddit who dropped this nugget of wisdom several years ago. It was nice to know I wasn’t dying or whatever.
Oh hey… I used to get those all the time as a kid!
Eventually I worked out that it could be made to instantly go away if I stopped breathing and just stretched my shoulders back and twisted in the same direction that the cramp was on
It always felt like a stabby, stabby muscle twinge right under my rib bones.