For once I can’t call what’s up with any certainty. If you’d humour me could I ask a few more questions?
Were many tests performed on you? Do you remember what they were?
Neurological symptoms? Numbness, tingling, loss of sensation e.g. at some of your fingers?
Location of back pain: The shoulder blade is pretty big. Is your pain unilateral near midline near the top of your shoulder blade (or bottom of the neck) or lower?
Shoulder Abduction, internal rotation or external rotation weakness?
Am thinking nerve but you’ve no pain or other neurological symptoms aside from weakness.
Brachial Plexus injury
Nerve root injury/impingement
Thoracic Outlet Syndrome
May not be nerve related e.g pectoralis major injury.
Anyways you can see that without some examination and imaging we’ll not know exactly what’s going on.
Monitor symptoms for now. If there’s change probably best to inform your healthcare professional.
Keep us informed and good luck