Are you hypermobile?
I get that and it turned out to be a stiffness/subluxation where one rib was attached to my spine. Sometimes it can crunch back into place naturally or with gentle stretching. Another time it needed to be adjusted by a physio. (Avoid chiropractors.)
SaneMartigan@aussie.zone 5 days ago
Subscapularis and serratus anterior are the muscles in the area. Could also be rib head / vertebrae articulations.
Poi or club bells are good shoulder mobility exercises. Else time heals most things.