Discovery reversing and not paying a preapproved claim

Discovery paid a hospital claim for me plus a portion of the specialist. All bills were settled in full by both discovery and myself then my gap paid me back the balance. I then 3 months later received an sms from the doctors rooms stating an outstanding amount that Discovery reversed without notifying me! I phoned them and they told me it was an error however the claim was approved upfront (in hospital procedure) and processed and paid by them. How can they just change their mind? Now I can’t claim the amount from my gap cover and because discovery has reversed a portion of the claim which was pre approved, the gap cover is Giving me a hard time. This puts in a very difficult place financially and I can’t believe discovery can get away with this! A pre approved claim, paid and then reversed months later?? That was supposed to be for discovery’s account. I am thinking about changing my medical aid and cancelling my discovery credit card because of their actions. Large corporate just doing whatever they want!