Thanks for asking the query,
After Root canal Treatment
the canals are completely devitalised and sealed to the apex, so there are no chances of pain.
Pain can be due to incompletely done procedure, accidently left over canal , re-infection, food lodgement between cap and tooth if the tooth is capped.
Get an x-ray done which will show any remnants of pulp tissue.
Depending upon the x-ray the treatment will be either re-RCT or extraction of tooth.
Take complete course of antibiotic and analgesics.
Hope this helps out.