Hello , It is nothing" Cervical  attrition" Medical terminology means enamel around that area is teared off and the second layer of teeth that is dentin is got exposed , which will gives you sensitivity  while drinking hot and cold beverages  ,  it is due to bad brushing technique . Also i can see some stain on affected area . Visit to your dentist for Treatment. 1st He / She will remove that stain with Ultrasonic tip under Local anesthesia  followed by Cavity and Composite filling . 2nd Ask your dentist  for proper brushing technique Use ultra soft tooth brush to avoid any tooth to get tear off ..

Hello dear Hope I can lessen your discomfort This is gingival recession that needs to be treated properly.etiology is rigorous tooth brushing. Than pain is due to root exposure I suggest you to follow proper brushing technique in atraumatic view

Follow correct brushing technique. And use soft bristled brush. Quit habits completely, get a scaling done and wait for 6 months.

Is there any swelling please tell? As for now brush slowly , harsh brushing for long can result in this. Go for a scaling and root planing this will deep clean your teeth it will help