Thanks for asking the query,
After successful Root Canal Treatment the canals are completely devitalised , disinfected so there are no chances of pain .
Pain can be due to incompletely done procedure , left over unexplored canal , re-infection , food losgement between the cap and the tooth if the tooth is capped .
Swelling indicates spread of the infection to the periapical tissues leading to formation of abscess
Take an x-ray of the tooth , which will show any left over remnants of pulp tissue or periapical infection depending upon which the treatment will be either re-RCT or extraction of the tooth.
Get scaling and polishing done .
Take complete course of antibiotic and analgesic
Use saline water gargles , use antiseptic mouthwash gargles twice daily.
Hope this helps out.