Thanks for your query, I have gone through your query. The pus discharge from the mouth behind the front teeth can be because if the infection in the tooth or gum resulting in an abscess. Nothing to be panic, consult an oral physician and get it evaluated. Mean while you can take a course of antibiotics like amoxicillin and metronidazole(If you are not allergic). You need to get a radiograph done to confirm the diagnosis. Once the diagnosis is confirmed, we can treat accordingly. Do saline gargling. I hope I have answered your query, take care.