It's a prolapse. I'm sorry it's happening to Choo-choo.
She
should be fine. The odds are it's a temporary thing. Since she's getting treated for bumblefoot, it'll give her a few days to relax and for everything to heal up.
There are things you can do, but the big one is to make sure she's not getting that prolapsed tissue pecked at by the other
chickens. It's red and unusual and a
chicken's first thought is to peck at the New Red Thing. That can lead to Badness. Lots and lots of badness.
Since she's a personal flock member, and it doesn't really effect her laying or chickening around, she should be fine. As Rebecca noted, it's caused by a whole bunch of things, and the heat that's hitting a large part of the world right now just puts the hens under more heat stress so we're seeing more of the more common stress-related things. That's what this is generally considered.
It doesn't have anything to do with her bumblefoot. Since it retracted on it's own, it's not even a serious issue, though I would keep an eye on her for a couple of weeks. It might make a reappearance at an inopportune moment.
Otherwise? She's cool. Since you'll be checking on her for her treatments, just keep an eye on her for that, too.
I'll think healing thoughts in her direction!