Hello ,
by watching the photos that you have shared , it is found that a crown is present at your lower left first molar tooth . If the ulcer has occured after that crown insertion , then visit to your dentist for check up the crown that has been placed . Or else , if you are stressed about some matter , try to reduce the stress also.
Next Steps
Consult with your dentist for evaluation of crown.
You may use Amlexanox with Lidocaine gel .
Health Tips
Try to avoid spicy food for at least 7 to 10 days.