hello and welcome,
According to your history i suspect you have got ulceration of the oral mucosa
which is caused due to consuming too hot food stuffs or it can be allergic lesions.
Exact diagnosis can be made only after clinical examination so get it evaluated by a dentist
Meantime you can apply dentogel , an antiseptic oral ointment over the ulcerated area which helps in healing.
use lukewarm saline water or chlorhexidine
mouthwash for mouth rinsing ill give soothing effect.
start with multivitamin
supplements 2 time daily for 5 to 7 days .
if the conditions does not get better with this , you need to approach a dentist.
i hope this will help you ,