1.ensure you don't have medical condition. -
Diabetes mainly.
2.if it's limited 2 one teeth then most common thing could be food lodgement.
3.If old filling/crown then an xray may be required.
Next Steps
floss 1st and salt water /Betadine rinse. wait for 1 or 2 days.if continues see the dentist.may require a xray and evaluation.
Health Tips
3- Fs. floss,flush,finger massage.