I had this same injury, it's a bitch bc it lags and hangs around til it gets to the point where it's excruciating. I think mine was caused from over benching, and having flared elbows. I have a very long wingspan (6'8") so I would get this aching dull pain under my armpit/inner bicep. I fixed my form on bench elbows pinned, half reps only and worked on external/internal shoulder rotations with bands and it gives me no pain today. good luck man sometimes your body just needs a rest I did trx workouts for 2 weeks so I was still able to stay active cuz I missing the gym for more than 3-4 days is like heroin withdrawal
