What Causes Recurring Mouth Ulcers?

Hello to all. I got a severe problem of recurring mouth ulcers. It s been more than a year now. My ESR (WINTROBE) count shows 45 mm in 1 hr. Rest here in DLC - Polymorphs 59, Lymphocytes 36, Eosinophils 02, Monocytes 03, Basophis 00, which indicates perfect. Total Leucocyte Count (TLC) 7850 is perfect. Haemoglobin is 12.0. Then what could be the possible reasons for recurring mouth ulcers? Or what kind of infection am I having for those of mouth ulcers? Please advise me.