Sick goldfish with big red butt and white in the middle

[Kaya]

My goldfish has gotten a small red bump on its anus about 2 weeks ago, but recently it has gotten much worse. His anus is half a centimeter big with white inside. He is pooping and eating though, but i think his fins have gotten darker. Im new to fish so i dont know whats going on. I talked to the local vet and they said that the fish was probably sick before i even got it ( i got it 20 days ago). My other goldfish is completely fine