hello there ,
According to your history i can suspect some remnant of the infected pulp tissue has left unexplored during the root canal
procedure or there is a missed canal due to which the infection has gone worse and you have developed a periodontal abscess
due to spread of infection to the underlying tissues.
The symptoms of pain and swelling needs to be subsided by a course of antibiotic and analgesics before starting treatment.
The infected tooth can be treated by root canal procedure to devitalise it permanently and save the tooth or of the prognosis is poor the tooth can be extracted.
get an x ray
done the affected area and approach a dentist
for the treatment.
meantime you can start with antibiotic augmentin
625mg with analgesics two times daily, this helps in subsiding the swelling and pain.
i hope this helps,