You may never get a real diagnosis because the flu virus you had is likely gone, leaving the damage to your sinuses. Your doctor wants to rule out other possibilities that could be aggravating your sinuses further, now that they are damaged. There may not be an easy solution so it is up to you to find the treatments that give you some relief, and work with your doctor to rule out any other causes. We all learned with Covid that these viral infections can leave us with long lasting changes to our body, sometimes affecting many organ systems. Treating the symptoms may be the only option, and allowing for time to help with healing. I hope you find some relief with the changing of the seasons.