Around and under my left shoulder blade is grinding and it hurts, what exactly is grinding?

Not sure : Could be a problem with the shoulder mechanism. May want to consider a visit to your primary care for an exam. May need an orthopedic referral as well. Hope this helps!

Varies: There are a number of potential causes. Some people will actually have a osteochondroma in the undersurface of their scapula that can cause grinding as it rubs over the rib cage. Their are many other possibilities. I would not hesitate to be seen for a specific diagnosis.