hi and welcome,
i suspect you have developed a gum abscess
on the tooth where the root canal was done.
This abscess can either be tooth related due to infection in the tooth that drains via gums or
it can be related to periodontal tissues, that is ginvival abscess caused by poor oral hygiene
This can be confirmed by taking and x ray
of the affected area. so vist a dentist
and get a x ray examination done .
accordingly, if it is a gum abscess , it is drained by currettage , deep scaling and root planing of the tooth is done followed by a course of antibiotics will subside the symptoms.
If the abscess is related to tooth , root canal treatment
has to be repeated.
i hope this helps,