What Could Cause Burning Sensation In My Elderly Mother’s Mouth And Throat While Eating?

My mother - age 96 - complained yesterday and today about burning pain in her mouth and throat when she eats, as if she’s eating spicy food like chile or has burned her mouth with hot food, even though she hasn’t done either one. Her tongue is a bright red, no white patches. No fever. She wears dentures which are old and ill-fitting. I’d like any suggestions of what it could be or what to do. (Milk soothes the pain. Her prescription meds - losartan, amlodipine, and omeprazole - have not changed for 2 years.)