Yes doctr is correct..better u cn go for re-rct as we can see infection n the root end portion.. actually the pin like thing which they hv placed inside ur teeth should reach almost to the end of the root,but in ur case it has not reached its short of length,so some infection was left behind and n due time it grew up to become a big one and that's the issue now.
Next Steps
consult ur endodontist and get a re RCT done.re rct Wil be bit more costly than normal rct
Health Tips
do place a cap above the RCT treated tooth after treatment as it become weak after treatment and a there r more chance for it to break