Middle left side back pain

I have been having pain in my back left side middle for about 3 weeks. At first I tried heat, cold, naproxin and was checked at urgent care for kidney problems and took a round of antibiotics and used lortab for about a week. Problem has not gotten better, but does seem worse in the morning. I have a really good mattress (Posturepedic), and massages don't help. I feel like the problem is "inside" rather than muscle or bone. I am obese. I don't have insurance.