Hello. A feeling of relapse immediately following removal of braces is something that can be expected, as our tissues have a tendency of trying to go back to the last stable position. Rest assured, it' ll definitely not undo all work that you have achieved through treatment with braces.
As soon as you start wearing a retainer, your teeth should stabilise into their new position. In case there is actual relapse happening, your orthodontist can always use an active retainer to correct minor relapse.
Next Steps
I would recommend , you discuss your concern about teeth relapsing with your orthodontist as he'll be best able to evaluate and advise you and probably expedite the fabrication of the retainer.