Why does my back hurt when I take a deep breath?

I have been on two antibiotics and a steriod inhaler. I am 18 years old and very fit, I play basketball about every single day. I have really bad allergies and have pretty bad drainage because of it. I have had an x-ray to check for pneumonia, and it was negative. It has been hurting for probably about a month now. When I take a deep breath, it hurts in my upper/mid back on the left side. It kinda felt like my lung was sore to start with, but now i think it is mainly my back. What could this be??