Simmons Mattress Beautyrest Black Rosalyn Mattress back and shoulder pain..... Atlanta Georgia

After the Hurricane Ike flood, we purchased a Simmons Beauty Rest Black Rosalyn Mattress....thinking we deserved the best after sleeping in a 35' trailer for 9 mo. and 22 days. By day 3 my back was killing me. I called the mattress store to see about an exchange or refund. They said the mattress we had purchased was the last one in stock, and no excange was possible. I bought a 4" foam pad and we used that till recently, when the constant strain on my back became too severe. I started started sleepig on the "Black" again, and nowam in terrible pain again from sleeping on a 'rock'. Cant afford another mattress, daughter will be going to college and hubby has passed away. Need help and dont know what to do.