What Causes Pain In Between The Spine And Shoulder Blade?

For about 4-6wks, after a weekend of golf I felt a pop in my mid thoracic region. It tightened up and ever since I have pain between my spine and shoulder blade just on the left side. The muscles there have knotted up and are tender at times. The pain varies throughout the day, is fine during some exercise but flames up during others. I thought it was just a rib that popped out of place, but adjusting it only offers temporary relief.