Removing braces, or removing the attachments used with aligners, makes the teeth temporarily more sensitive. This sensitivity is expected to settle gradually over a few weeks. Any oral hygiene products with a high fluoride content reduce tooth sensitivity and help shorten this period. You can use an orthodontic mouthwash, a toothpaste for sensitive teeth or an enamel-remineralising toothpaste. Your orthodontist may also recommend, or give you, a special fluoride gel to apply to your teeth to reduce sensitivity. On top of that, keeping your teeth and gums perfectly clean will help any irritated gums settle down more quickly.
This answer is for general information. Your own case needs an examination by an orthodontist.