Help for my chicken

Welcome to BYC.
Can you post some photos?

From your description it sounds like your girl has a prolapsed vent.

Bring her in and give a her soak in a warm epsom salts bath. Clean up the vent area and the prolapse (part hanging out).

Apply some honey or hemorrhoid cream to the prolapse, then gently push it back in. Sometimes once started she will retract it. If it comes back out repeat .

Keep then vent moist with the honey or cream. Offer her some extra calcium, you can use TUMS or liquid calcium.
The calcium will help with contractions to keep the prolapse in place (hopefully).

Keep watch on her, if she is trying to lay an egg or it's close to time to lay her egg, monitor her, she may prolapse again when she lays.
If so repeat the process.

Keep her separate for a day or so to make sure the prolapse will stay in. It can come out again, so just repeat your process.

Make sure she is drinking, if you have any poultry vitamins, add that to her water. Feed her normally.

http://www.the-chicken-chick.com/2012/04/prolapse-vent-causes-treatment-graphic.html

 

You did a good job in taking care of her. You followed your instincts and realized that needed to go back in. Just keep an eye on her for a little while, if it comes back out, then just repeat what you did.

I don't think the oats or cheerios would cause constipation, but they should be given in moderation as a treat. I do give mine oats as a "scratch", I just scatter a bit in the run, my flock loves it, but it's just small amount. Cheerios are most likely ok, but just a small amount as well. Fresh fruit/veggies, weeds, etc., are also good to give. Mine are very partial to Kale (hang it up or place it in a suet basket), so I keep a good supply on hand.

Make sure they have source of grit - either commercial poultry grit (crushed Granite) or have access to dirt where they can find small pebbles on their own.

Not drinking enough water can cause some constipation. You can also mix some molasses or epsom salts in her water this will act as a laxative. Remember, a little goes a long way, so I would only give just a little "laxative" then encourage plain drinking water. (Recipes below - just break the amount down into smaller dosages)

Prolapse can be caused my many things, so offer oyster shell free choice, limit treats - obesity can cause problems and sometimes it just happens.


You are welcome for the help.

Keep us posted on her progress.

LAXATIVE SOLUTIONS
The following solutions or mixtures are recommended to flush the digestive system of toxic substances, most notably for treating birds exposed to botulism toxins.
Molasses Solution

Add one pint of molasses to 5 gallons of water​

Offer the drinking solution free-choice to the affected birds for about four hours. Treat severely affected birds individually if they cannot drink. Return the birds to regular water after the treatment period.
As a supportive treatment for symptoms resulting from Cryptosporidia infection, often referred to as coronaviral enteritis, use:

One quart molasses in 20 gallons of water​

Offer this solution free-choice for a period of up to 7-10 days. It is assumed that the molasses replaces certain minerals lost from diarrhea during the course of the infection.
Epsom Salt Solution

1 lb Epsom Salt per 15 lb feed
-or-
1 lb Epsom Salt per 5 gallons water for 1 day​

Give the epson salt feed mixture as the sole feed source for a one day period. This feed can be used only if the birds are eating. If the birds are not eating, use the water solution. If the birds are unable to eat or drink by themselves, use individual treatment with:

1 teaspoon of Epsom Salt in 1 fl oz water​

Place the solution in the crop of the affected bird. This same amount of solution will treat 5-8 quail or one chicken.
Castor Oil Therapy
Dose individual birds with .5 oz castor oil.
REFERENCE SITE: http://extension.msstate.edu/content/solutions-and-treatments