Hi....
What I can see from the picture is not stone, stone would have caused you a lot of pain before and during chewing your food.
You would have had a cheek bite which is very normal.
The reasons could be distractions during eating, tooth deflection in the arch, biting during sleep, psychological reasons.
It can happen again but if it is very frequent then you’ll need help and then you’ll have to take it seriously as it could lead to infections.
Next Steps
Try applying antiseptic oral gels, use ice cubes wrapped in a cloth and put it on the site for 15-20 mins thrice a day, you can use pain killers if you have pain.
Health Tips
Consult a dentist online / offline soon