Their is a difference between epulis and parulis: Epulis is a tumor like enlargement on the gingiva. Parulis is gumboil (it contains collection of pus). Generally in a RC treated tooth if their is recurrence of the infection, a gumboil on the gum occurs which is an abscess. Without picture or radiographs it's hard to differentiate and give advice!!

X ray

Get xray of tooth. Re-rct of tooth has to be done if required. Take antibiotics along with analgesics to relieve pain and swelling. You can consult through this for prescription of medication.