What may be causing the pain in center of my back behind my heart?

I have this reoccuring pain in the center of my back between my shouldblades. It comes and goes. Mostly after I have eaten or in middle of the night. I thought it might be the start of gall bladder problems, but now I am not so sure. Has anyone else had this and can tell me what it is. I can't afford to go to the doctor unless I absolutly have to.