I've Got A Pain In My Left Side Chest Wall

I've got a pain in my left side chest wall about 6 inches below my armpit. It feels like an overzealous massage therapist or martial artist has their thumb stuck in between the ribs. Measured from my Samsung Health app my HR is 48- 2nd measurement shows 49. Oxygen saturation shows 98%. I feel it more the deeper I breathe and completely without any of the classic "heart attack" symptoms of chest pressure or any radiating pain. On a scale of 1 to 10 it's a 4 to 5. I keep moving to try and get it to dissipate but it still feels like I've got someones thumb stuck in my ribs... what is this madness?