Sharp Mid Back Pain from Volleyball

In volleyball practice the other day i went up to hit the ball and felt a cracking sensation in my mid back on the left side a few inches under my shoulder blade when i hit it. I immediately felt sharp pain and the wind was knocked out of me. I continued to play with the injury for a few days with no improvements. It has now been 5 days of not playing on it and it still hurts and doesn't seem to be getting much better. For the most part it only hurts when i swing my arms up in the air as if I was going to hit a ball. Anyone have any idea what it is? Thanks.