Hi.. Can understand your concern.. As per your complain swelling and pus discharge from the gum is a sign of gum infection leading to inflammation and the condition is known as Gingivitis.. Flossing can cause trauma to the gums if gums are injured and can get infected if the debris gets deeply impacted into the gums leading to pus formation.. You need not to worry and just consult a dentist and get professional cleaning known as Scaling done along with antibiotics like amoxicillin and metronidazole and antimicrobial mouthwash can cure the infection and inflammation.. Hope this information helps.. If you find this answer helpful kindly write a positive review and click on found this answer helpful as a token of appreciation.. Thanks and regards.. Dr. Honey Nandwani Arora..