Peculiar shoulder sensation that mostly comes up when I relax?

Just curious if anybody has cursory experience with what I will describe.

I have never had any serious or notable shoulder injuries that I can recall.

I notice this mostly when I am sitting relaxed, and exhale.  The muscles in my shoulder relax and then I feel...not a tingling sensation like a muscle falling asleep, but a sort of...crackly sensation?  Not really electric like a pinched nerve, I don't think, but close.  The sensation is on both sides of my armpit, like where the pec meets the pit and then the meaty lat on the back of the armpit. I can occasionally feel the similar sensation when I do certain exercises.

I am rather flexible and have found no way to engage or stretch out the sensation other than when I breathe out.  It is only in one shoulder.  Is it just a lightly pinched nerve?  Very curious.

I highly recommend you get diagnosed by a physio. I have notified Mark and Wes about this so they may have some advice but nothing beats an in-person diagnosis from a good physio.