Hello ,
There could be re- infection accosiated with rct treated tooth or may be deep cavity in adjacent tooth of rct treated tooth .
Need to rule out endo - perio ( Tooth as well as gum lesion ) lesion associated with both teeth .
Need clinical examination for periodontal pockets , cavity , POP / TOP test .
Need Xray for proper diagnosis.
Next Steps
Visit your nearest Dentist for clinical examination ,X ray and for treatment .