Hello sir... Your clinical picture shows mild gum swelling... It may be due to re infection of the root canal treated tooth because of failure to put capping of that tooth  or it may be carious infection of second molar teeth which causes infection periapically around the root....   It may be pocket formation between 2 molar teeth..... However complete clinical and radiological examination is necessary to rule out the cause and management... If it is due to re infection of RCT treated tooth then re RCT is necessary followed by capping.... Or due to infection of the 2 nd molar teeth then root canal treatment is mandatory

The gum appears swollen which suggests infection. Reasons for this could be many. I suspect for pockets between your teeth. Could also be a cavity in the adjacent tooth or a faulty RCT. Also if it's RCT treated.... No crown has been placed... RCT is incomplete without crown.

Please upload an X-ray if u have any!

By looking at the pic there is lot of debris in between your last and the second last tooth. The pain may be because of reinfection in your rct tooth but most probably i guess it is due to cavity in your second last tooth ( it is looking slightly black from the one side and also there is a small swelling in the gums which may be due to pus formation). This is only confirmed by proper examination and x-ray.