Pre-approved then denied claim

I was admitted into hospital as i have hospital cover. I needed 2 mri's done and was told one was approved. I was also told by a representative of discovery, a lady who came to my bed and said everything in hospital has been covered by my hospital plan besides ONE mri which i'm paying cash for. Now i have been told that no mri's are covered and should not have been pre-approved as my medical aid does not cover it. More claims are now coming in and they are denying them all, saying i have no savings account... I was in hospital for 6 days.. How can none of this be covered in a hospital plan. How can discovery give approval to the doctors and the hospital to do tests and scans then when the bills come in they deny the claims!!! I am now stressing to the point of breaking down as the claims are reaching R20 000.