I never heard of it so I googled it and I didn’t find anything that sounded like what you are describing, do you have a link? It sounds to me as if it could be a hemorrhoid, or just a pimple of some sort. Sometimes and abscessed anal gland will cause an external symptom but I am not sure where exactly this little lump is. Do you trust this vet, have you had other satisfactory dealings with her/him in the past?