It's might be because of gum disease called pyorrhoea or pericoronitis.
So for exact diagnosis can you share clinical photograph of the teeth in question? Also, if possible you can consult on Practo for further information about it.
Next Steps
consult online through Practo for further information about it. Let me know, if I can help you to find out best solution for it.
Health Tips
following proper brushing technique with toothpaste will maintain oral hygiene & rinse with lukewarm salt water after every meals to maintain oral hygiene.
Thanks!