In most cases the cause is not known , but sometimes it is due to one of the following reasons: 1. Iron , Vitamin B12 or folic acid deficiency 2. Women before menstruation , after menopause 3. Stress or anxiety 4. Any Side effects of drugs such as anti-inflammatory drugs , nicotine therapy (oral) etc. 5. Allergies 6. Hereditary 6. Various diseases , such as celiac disease , Crohn's disease , HIV / AIDS , colon cancer etc.