Like the other folks have said, it does sound like ascites whether the cause was peritonitis or internal laying. You can also see it with ovarian cancer, though your girl sounds young for that. Other infections can also cause it, but with the production Rhode Island Red strains I've kept it's always been their reproduction system gone awry.
My avian vet does work with 'geriatric' chickens, which unfortunately with the high production breeds, can mean a bird that's still very young. At least as far as the birds she's seen, it's usually been peritonitis or internal laying in birds under four years and cancer in birds over that age. She did an ultrasound on one of my four year old Reds who had similar symptoms and the poor gal was just full of unlaid eggs.
From what I've seen, it absolutely runs in at least the hatchery strains of Rhode Island Reds. I've kept Reds from several different sources and lost every single one of them to some reproductive related malady at five years or under. Despite how much I love them, I don't keep the breed anymore for that reason. My birds are pet as much as they are layers and until I find a local source for a non-production strain, the Reds are just too much of a heart breaker for me.
There isn't generally much that can be done for the condition, and some with don't stay around long, but sometimes you get lucky and their laying system shuts down, at least for a little while. Like that Red I had that the vet found all those eggs in - she was with us for almost another year. She wasn't as fast as the other gals, but she managed to keep up well enough, did all the normal chicken business in her own modified way (she did also use her wings for support at times), and was spoiled rotten as long as she was with us.
I hope your little gal keeps doing alright in her own way!
