What Causes Pain In Midriff Area And Back?

I had to call an ambulance yesterday due to the severity of pain around my midriff and back. I was given morphine in the ambulance (twice) which eventually eased the pain. I had blood tests ecg chest X Ray and urine test and all were clear. The pain eventually went all togethter. I was told I could go home but with no diagnosis. I am now afraid incase the pain comes back. I am getting niggles and tenderness in the same place but no pain Thankyou