Ulcers that come and go within 7 days are called aphtous ulcers, they are often related to physical and mental stress. A healthy life style can reduce them,  eat around 400 g of fruits and vegetables a day. Even if you are vegetarian you need to ensure a mix of various vegetables and pulses. Ensure you get 7- 8 hours of undisturbed sleep at night. Try deep breathing exercises. If you are not relieved, visit a dentist for exact diagnosis and prescription of topical applications that will reduce pain.