What is this throbbing under my shoulder blade?

Complex Joint: The shoulder is a complex joint, & many conditions/ injuries (rotator cuff injury, bursitits, tendonitis, labral pathology, etc).Can cause pain/ weakness. Some injuries of the soulder occur due to acute injury and some are from overuse. Ii suggest you see an orthopedist for a complete exam.

Varies: There are multiple potential causes, among the more common is a subscapular bursitis. Don't hesitate to be evaluated for a specific diagnosis and treatment options.