Hi, it could be due to decayed tooth. You will surely have to visit a dentist for a consult and get an X-ray to determine its severity and line of treatment. If it’s a simple decay can be treated by removing the debris, cleaning and filling it with a restorative material or if it the decay is deep enough you will have to get a root canal done. Or if you could share a picture of it, we can discuss in further detail over a video consult.
Next Steps
Brush your teeth with a fluoridated toothpaste once in the morning and once before going to bed at night. Gargle after every meal. Use mouthwash and floss in your routine oral hygiene regimen.