hello & welcome to HCM,
I just went through your problem & it can be apthous ulcers.Common Causes can be:-
-Improper food habits like taking much of spicy food
-Improper Oral hygiene
-Lastly if its reccuring then it can be due to some sharp cusp.
What all you need to do is improve her dietary habits like include more of green vegetables & less oily & spicy food.
Also start with Tenvit-Plus capsules along with application of some anaesthetic gel like dologel or Mucopain gel.
You can also go for tetracycline
All these will definitely help you out.