probably a prolapse. clean it off with warm sterile water, get as much debris off as humanly possible, and if you use gloves with a little bit of Vaseline you can push it in, careful to not cause any damage. you may have to have a vet suture the wound so it does not happen again.
I'd have to talk with my parents, they don't really care about the chickens as much as I do, and I hardly ever use my phone, as I need permission to do that, and they never give me permission. I'm thinking it may have something to do with her suddenly going broody?
I know this is an older post, but could it have been a breast blister? hens typically get them from irritations on their roost, like if its dirty or there's little wood splinters poking up if you have a wooden roost
I'm so sorry that you are going through this. I had this loving blue Americauna hen named Herobrine--all the positive traits you described above--and she was my best show bird. (won 3 blue ribbons, one of which was a third blue championship out of 168 people) i had only shown for one year, and...