One of our cats has a similar sounding bump just above her anus. The Vet says it is a "dog bite adenoma". It is full of blood but is not irritated or bleeding. Bitty, also age 12, has had it for several years but it is slowly enlarging and we have decided now is the time to have it removed, before it gets any larger. I'm waiting for them to call me with the surgery date.
Here's hoping and praying all goes well for your pup and my cat.
Hemorrhoids? I asked and my Vet said dogs and cats don't get them because they don't strain as humans do. I don't believe everything I'm told but I never got around to looking it up. Do cats and dogs get them? They certainly do strain sometimes.