How To Identify and Treat Prolapse in Your Hens
It is very difficult in the first instance to tell or diagnose when a chicken reaches maturity if it is going to have any internal issues a sometimes can be somewhat of a surprise to new chicken keepers when chickens fall ill, but judging from your conversation it seems that something is protruding from the vent area, which as you suspect is likely to be a prolapse, this is where the oviduct or egg laying tube has been pushed out of the body in an effort to lay an egg. It is vitally important to separate her from any other hens you might have as they will be attracted to the redness and peck at it causing your hen distress and serious injury.
To treat the prolapse, first wash the prolapsed area with warm water with some antiseptic in it. Dettol or Savlon will be perfect. You may see that there is an egg in the prolapse and you'll need to carefully try to remove this without breaking it. If you do happen to break it, make sure that you remove any pieces of shell as they may lead to an abdominal infection. Once the prolapse is clean, tuck the hen under your arm with her head covered to keep her calm and gently push the prolapse back into the vent cavity with clean hands or wear clean rubber gloves or even use a clean wet cloth to help you push it back. Keep the hen in a darkened room or box to recover and allow the prolapse to settle back into her body. Many people recommend smearing honey onto the prolapse as this has antiseptic and healing properties. Applying haemorrhoid cream to the vent afterwards can also help. You will need to stop her laying for a while to stop the prolapse from just popping back out again so keep her off her layers meal for a couple of days and only give a very bland diet such as wheat (weetabix mixed with water or Growers Pellets) and if she's kept in a darkened room for a few days, this will also stop her body from being stimulated into laying. She may need veterinary treatment if it keeps happening and if it does, I'm afraid that the prognosis isn't good.
Making sure that she has a good intake of calcium may help as this strengthens the muscles in the oviduct and will help her to pass the egg. Limestone Flour mixed with her layers meal will help boost her calcium levels and you can get this from animal feed suppliers in the equestrian section, you add a heaped teaspoon of this to a feeder of pellets or mash a couple of times a week.
Also giving lots of leafy green vegetables can help with the absorption of calcium into the body so would be a good treat for her, cod liver oil given in moderation can also help with the absorption of calcium. If you give a a tablespoon of cod liver oil over a week, this may also help. Don't overfeed your hen with other treats though as prolapses are more common in overweight hens.