What Causes Lower Back And Abdominal Pain?

I had ct scan that came back inconclusive on colon and abdomen but I am still having same pains in left side lower back and up inside me when I use the bathroom I have a lot of mucus I am worried that something is seriously wrong the hospital sent me home with a letter for doctor to wait for a colonoscopy but pain is not getting any better and I have a lot of pressure in my back