An anal fistula is a small tunnel that forms between the inside of the anus and the nearby skin. It usually starts after an abscess bursts or heals poorly, and it keeps leaking like a broken pipe. Ongoing drainage is the hallmark. You notice yellow, green, or bloody streaks on your underwear that return every day, even after a shower. Pain comes and goes. It feels like a deep, dull throb during bowel movements and eases when the tunnel drains. Swelling pops up. A pea-sized lump near the an opening opens and drains, then shrinks, only to swell again. Itching... Learn more