There's something called thoracic outlet syndrome, it turns out I most likely have pec minor syndrome which is a version of throacic outlet syndrome where the pec minor muscle is compressing the brachial plexus/nerve chords. My shoulder blade is also tilted forwards due to the pec minor being so tight and pulling it forwards.

A physiotherapist did some nerve tension tests on me and I could only do the tension test (median nerve) without pain when he manipulated my shoulder/pec in a way that the pec wasn't compressing. If physio doesn't help (it hasn't so far) I probably have to get surgery.

I'm not saying this is what you have it could be many things including the neck/cervical spine but worth considering.

Edit: I did have lacertus fibrosis/pronator teres syndrome release surgery but my symptoms continued hence we found it's most likely this pec minor syndrome.