This is likely to be aphthous stomatitis. Treatment are following;
1) Benzocaine or Lidocaine gel
2) Amlexanox paste
3) Multivitamin, multimineral; heals them and prevent recurrence.
4) Avoid spicy food.
5) Avoid any kind of chillies.
6) Fruit juices and fruits are good.
7) Breads / toasts / Gems can be given.
The cause of aphthous stomatitis is not clear and is not known. It is suspected to be due to stress, genetic predisposition, local infections, nutritional deficiencies, intestine disorders, food allergy etc.
The other cause can be viral infections. Treatment is same and is a self limiting disorder.
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.