Hello and welcome to HCM,
Your tooth is root canal
treated so cannot suspect any problem with the tooth. vigorous flossing must have injured your gums leading to gingivitis
,that is inflammation of gums which causes redness and swelling. you can have tablet metrogyl along with analgesic
3 to 5 days. mouth rinsing with lukewarm saline water or antiseptic mouthwash helps in quick healing and gives soothing effect, apply gum paint over the gums after food. After 3 days if pain persists visit a dentist
and check for any broken filling or adjacent infected tooth. take care.