hello there , welcome to HCM, The clinical signs you have mentioned are suggestive of Oral mucositis , that is inflammation and severe ulceration of entire oral mucosa,it is caused either by streptococcal infection or apthous ulcerations. It cannot be oral thrush as the candida fungus appears as white pathches. It may be caused due to several reasons like deficiency or certain vitamins especially vitamin B12 and vitamin C. start with multivitamin supplements for 7 to 10 days and vitamin C lozenges will help in resolving the problem. avoid alcohol , carbonated drinks , tobacco chewing and smoking as these may worsen the area affected. Apply oral antiseptic ,analgesic ointments like dentogel or mucogel topically over the affected area. Maintain good oral hygiene and drink lots of water and keep yourself hydrated. regular mouth rinsing with antiseptic mouthwash or lukewarm saline water gives soothing effect . If the symptoms do not subside with this , you need to go for pathological tests like biopsy. i hope this helps , take care.