Good evening! You may have cavity or reduced layers of normal teeth. Brush your teeth with Oral-B UltraSoft Tooth Brush in circular motion without applying much pressure. If there is cavity kindly consult a dentist and get filled. Thank you!
Kindly get an X-ray done of the tooth having pain .may be you will have to get root canal treatment done followed by a cap /crown. Brush your teeth twice daily using correct brushing technique to maintain your oral hygiene.
It may be due to sensitivity ,u should apply fatkiri to your gum, rinse your mouth with warm salt water, chew few leaves of guava, boil few leaves of guava and rinse your mouth with that water, visit your dentist for check up.