What is this pain behind my shoulder blades?

Hmmm. I think you need to describe it more.

It could be: Stress, position at computer, thoracic and/or cervical vertebrae out of alignment, lower back out of alignment, muscle spasm, recent heavy lifting...

When does it hurt?
What are your usual activities?
Describe the pain (knifelike, aching, pulling, burning etc.)
How old are you?
Have you seen your doctor about it?