Thanks for asking the query,
can be due to improper oral hygiene
and calculus deposition, vitamin deficiencies, systemic conditions.
It can be a food particle stuck in between the teeth, try to remove it with the help of sterile dental floss.
If not then visit to the Dentist
and get the checkup done,
Go for complete mouth scaling and polishing done.
In the clinic it can easily come out with the help of instruments like mouth mirror, probe.
Maintain a good oral hygiene, use antiseptic mouthwash gargles twice daily.
Hope this helps out.