Ouch, you're eating something and you accidently bite the inside of your cheek. The next thing you know, you have a painful mouth ulcer. It's tender, it's swollen, you have problems chewing or brushing your teeth, and you can't eat spicy, salty or sour foods because it hurts so much. Often, it's difficult even to speak.
You can get mouth ulcers from brushing your teeth too hard; burns from hot foods or drinks; sharp, rough teeth/fillings or rubbing against orthodontic braces; or even a mere slip of your toothbrush while brushing your teeth. Sometimes ulcers crop up if you have an infection, are under stress or when you have your menstruation. If you speak to your grandmother, she would probably say you're 'yeet hay' - you have too much hot air in your body because you've eaten too many deep fried and oily foods.
Most of the time, mouth ulcers are harmless and heal by themselves in a few days to a week. If they don't or if you keep getting mouth ulcers, then you'll need to talk to your doctor about it. Sometimes a lack of iron or vitamin B12 is the cause of recurring ulcers.
In the meantime, to ease the pain and speed up recovery:
Avoid spicy and sour foods until the ulcers heal
Drink lots of fluids, soups, water, milk