Thanks for sharing your concern.
The flat sore inside mouth can be of aphthous ulcer type.
This may be caused due to-
Poor oral hygiene
I would advice you to visit a dentist
and get a direct clinical examination.
Chance of the lesion being malignant has to be ruled out thro-biopsy.
Meanwhile-Maintain oral hygiene well.
Take plenty of fluids and vitamin rich diet.
Avoid anxiety and stress.
,iron and folic acid
supplements can be administered.
Hope this helps.