congratulations on the completion of your orthodontic treatment!
Our teeth are constantly moving, and people tend to notice that the teeth may "move more" during orthodontic treatment, or within a few months after orthodontic treatment.
The reason is because during orthodontic treatment or after, the periodontal ligament that cushions/holds the teeth in place likes to move back to its old position.
Several of my patients will feedback that upon not wearing the aligners or retainers for a few hours, the teeth may shift slightly, and upon wearing it again it feels tight on the teeth. The aligner or retainers will help to move the teeth back to the ideal position.
The periodontal ligament and bone around the teeth will take a few few months to be stabilized after orthodontic treatment.
The concern would be when the retainer doesn't fit, and a new retainer or another round of orthodontic treatment may be needed to correct the teeth that has since moved out of position.
Most of the time, the retainers issued by your dentist will be sufficient if you adhere to his instructions. You may try that for a start, and if the problem still persists, do consider consulting your dentist.
