You need a proper gas evaluation buy an oral pathologist so that the exact cause of these recurrent ulcers can be ascertained. There can be many reasons but right diagnosis cannot be established without a proper clinical examination and a detailed clinical history. The right treatment can be suggested after a rational diagnosis has been established. This can range from nutritional deficiencies or viral infections or fungal infections or extra oral causes like gastritis, etc. (And this is not an all inclusive list). You can post the clinical pictures of the ulcers on Practo for an online consultation with the specialist wherein a detailed clinical history can be obtained through some questions. Regards