I have middle back pain and pain under my left rib cage. there is an increase in pain when i take a deep breath, it is not muscular, what could it be?

Difficult to say: There are many possibilities here. The pain could be coming from the spine. I am not sure how you know it is not muscular. Other possible pain generators include the lungs, ribs or the internal organs. This could be a pain that originates exactly where you feel it or could be referred from a more distant location. You need a good physical examination by an md/do who is used to dealing with this.