Your x ray is not very clear. Please provide a good x ray for diagnostic point of view. Generally the RCT operated tooth does not cause any pain because all the nerves and vessels are removed from it . But it seems like there is some radiolucency around the mesial root of second last molar this might be due to some local infection or the RCT is not done correctly . Pain might be because of that. The filling which was removed is not a cause of concern you can get it done again . If the pain is very mild then you can tell your near by dentist to prescribe you analgesics or if you are having severe pain then book an appointment with your doctor to resolve the issue.
Next Steps
Donot apply too much pressure on that area . Donot clench the teeth with opposite arch.
Health Tips
Consult with the doctor either online or in clinic if required