How long ago was this?
Assuming you have a normal immune system your body will form antibodies to the virus and destroy it within a few weeks. It's rare for WNV antibody tests to be negative after 21 days.
You'll always test positive for the antibodies once you form them. That just means that you won't ever get infected by the virus again. The antibodies are ready to rush into action whenever they detect the virus and will destroy it. One of the reasons you got sick this time was because it took your body some time to form the antibodies and the virus was active in your system.
Follow your doctor's directions for recovery and activities.