Hello and hope you are doing well.
The ulcers could be canker sores. It is very painful and can occur due to stress, vitamin deficiencies or trauma. Apply chlorhexidine gel or any other oral anti septic gel you can find twice daily on your mouth ulcers and leave it on for 2 mins and then do warm salt water gargles. Drink or eat half hour after application. Take OTC multivitamins rich in riboflavin to counter the mouth ulcers. If it still persists consult your dentist.
Hope this helped and do keep us posted.