The reason for this discomfort is that the bleaching agent, whether it be hydrogen peroxide causes a chemical reaction on the tooth surface. This, in turn, increases the porosity of enamel and makes it more sensitive to hot and cold foods and drinks.
Next Steps
Use fluoride toothpaste for sensitive teeth twice a day after brushing to help promote re-mineralization.
use soft bristle toothbrush
it is best to avoid exposing them to anything too hot or cold
use mouthwash like colgate plax.
Health Tips
in case of further pain you can consult your dentist
stay safe