What Causes Swelling, Burning, And Occasional Pus Near My Anus Despite A Normal Rectal Exam?

Hi, I am having a problem on half portion of my anus. The anus half portion have swelling and there is no bleeding no constipation my general surgeon checks through rectai examination there is no Hemorrhoids no internal no external at start it with Diarrhea i felt burning then i clicked a picture and it shows me a boil. 1. on sitting there is no pain 2. No bleeding. 3. No abscess history. 4. No bleeding no pain after passing stool. 5. only burns when diarrhea occur. 6. It swells after physical movement like walk only half portion. 7. Sometimes there is visible wetty Pus/Mucus etc on Anus. 8. Digital rectal examination shows no symptoms of Hemorrhoids , When dr checks 360° through index finger no pain.