Help - sick goldfish

Hi, my daughter has an 11-year-old orange-and-white goldfish (feeder
type, not koi) who seems to be on his last...fins. He has been
sleeping tail-up between regular activity periods, and now she says
his whole rear end is red. He has no fungus or rot symptoms, and has
had a couple of episodes of a 'lump' on one side of his body/tail area
which we assumed was constipation, but she has seen him poop as
recently as yesterday and the lump seems to be smaller.

What might be causing of the redness?

Thanks
Jeannie