according to your explanation it looks like it is sinus tract and pus is oozing out through that,
Often swelling will come n go....
Usually it is chronic and Painless so mostly postponed..
Further reasons for failed RCT might be found via CBCT Scan which is 3 D study of associated area which may detect microfractures, extra canals, file separations, anatomical variations and dimensions of Abscess or cyst if any...
So better complete study first than taking any action
Next Steps
consult Endodontist asap
Health Tips
avoid eating hard stuff from associated tooth.