Greetings, I am Dr. Pat. I have worked with birds for many years. I will do my best to help you.

She may have fluid in the abdomen, peritonitis or other reproductive issues. Fluid is very easy for a vet to drain, and it can be analyzed to help come to a diagnosis.

She is in very serious trouble, and needs local veterinary attention. These signs are of a very sick bird, and not specific to any one disease. And that means it is not fair to you or her to guess, there are so many possibilities.You are going to need local help on this, and a scientific and solid diagnosis to find safe and effective treatment.

You can examine the bird thoroughly again, including opening the mouth and having a good look in there for mucus, redness, masses or anything else unusual. You can take the temperature gently with a rectal thermometer. Anything above 105F/40C is significant. Palpate the tummy for an egg, fluid, lumps or anything else. Check all the joints for swelling, pain, and mobility.

Move the bird indoors to an aquarium, box or carrier with soft towels or hay in the bottom, no perch, and food and water in low bowls that can be reached easily. Keep her partially covered, warm and quiet.

Do not try to force food or water. You can offer warm cooked rice, pancakes, cornbread, grapes, melon, greens in addition to normal food.

I know it is expensive, but you may not have many home options, because the first thing you need a vet for is to find out what is going on. Treatment is only as good as the diagnosis. If you call around, you may find a vet to work within your means. We certainly try to do our best in my clinic.

She needs to see an avian/poultry-experienced veterinarian ASAP for complete examination, diagnosis and appropriate treatment. Check

http://chickenvet.co.uk/

http://poultrykeeper.com/poultry-vets-uk/poultry-veterinary-practices-services-uk/

http://www.eaavonline.org/index.php?option=com_content&view=article&id=4&Itemid=10

http://www.parrotpassionsuk.com/Advice/Uk_Avian_Vets.htm

http://www.theparrotsocietyuk.org/index.php/Avian_Vets/28

http://www.avianveterinaryservices.co.uk/

http://www.birdvet.co.uk/
The expense for this is going to be a lot less than inefficient, ineffective, dangerous treatments, guesswork, and loss of the flock; not to mention possible implications to human consumption of tainted eggs. Many states/governments have poultry diagnostic labs that charge very reasonable fees to test for common diseases. In the UK, check:
http://chickenvet.co.uk/lab/index.aspx
Because you have others and presumably use the eggs, it is important to have a solid diagnosis and treatment safe for egg consumption. You can check with local County Extension offices, the closest Ag university with a poultry department, or with the closest vet school for a local referral.

If this were my patient, I would start with complete fecal analysis and direct smear, for multiple parasites; bacterial culture and sensitivity of the feces and choana. Depending on the case I might do a fungal culture. Routine blood work is necessary to rule out other issues. I would very likely order a number of DNA tests for poultry viruses as well. Generally I start them out on injectable antibiotics as indicated by the tests.

Pet/feed store medications and home remedies are harmful, ineffective, immuno-suppressive, and make them much worse and may interfere with the veterinarian's diagnosis and treatment. Do not use them.

The flock should be on a high quality pelleted diet with extra greens/pasturage. Of late, I have been recommending TOP pet bird pellets (TOP http://totallyorganics.com/t-pellets.php ) as they are more appropriate for the pet chicken. Overcrowding, cleanliness, proper water, environmental temperature, humidity, ventilation, photoperiod, and toxic exposures should be addressed. Check this
http://www.poultryhub.org/index.php/Welfare_of_poultry_in_periurban_environments for husbandry advice.

Check http://www.poultryhub.org/index.php/General_tips_for_small_scale_poultry_production for basic care, set up and maintenance. You need to check for fly and mosquito access, as they can carry certain diseases, and check for external parasites. Mites, lice and fleas (in some areas, ticks) can contribute to over-all health issues, anemia, and disease transmission.

She may need injectable antibiotics, calcium and many other medications. Act quickly and good luck.

A vet has the proper equipment and training to place a drain into the abdominal cavity, yes.