These recurrent mouth sores are usually caused by mineral and vitamins B12 or iron deficiencies. Especially due to low immunity. They can be also caused by virual infection which stays in your body and becomes active during low immunity. Start vitamin supplements containing folic acid, iron and Vitamin B 12 . If you live in pakistan you can get Surbex Z. One tab once aday. Keep yourself hydrated. You can get your Blood CP and feritin level done.