Unfortunately she is most likely suffering from a reproductive disorder which is pretty bad news. I guess you already know that though since you have lost two girls already with similar symptoms.
The commonest causes are:-
salpingitis .... an infection of the oviduct which gets packed solid with solidified puss and egg material eventually constricting the gut and causing waste to build up in the digestive tract. This makes the bird feel like they don't want to eat and sometimes the crop contents slow or stop and turn sour. Unless it is caught in the very early stages which is almost impossible to detect, it is not treatable and sadly will prove fatal. Usually it will have been going on for several days to a week or maybe slightly longer before it becomes obvious. The bird may have a bulge below the vent and possibly between the legs. Sometimes they will lay a lash egg (a lumpy flesh coloured mass that looks like a layered mixture of cooked egg and sausage meat) that may alert you to the problem. The birds will sometimes lose condition especially if they have not been eating for a few days.
internal laying... which is where egg yolks released from the ovary each day fail to travel into the oviduct and instead, drop into the abdominal cavity. The bird will usually look healthy for months with a nice red comb but eventually the mass of eggs in her will start to put pressure on her organs, often pushing her legs apart so she stands and walks with a wide gait. There will be an obvious swelling between the legs and under the vent which may feel like a water balloon (this is Ascites or water belly). They often start to suffer respiratory distress as the fluid puts pressure on their heart and lungs and again the mass will eventually constrict the gut, reducing or preventing the elimination of waste from the body. The bird may feel noticeably heavy when you pick her up as the mass of fluid collects in her belly. If this is caught early enough, there are potential treatment options to manage the condition which include draining the fluid off which will give almost instant relief but will almost certainly need to be repeated at intervals over the coming weeks and runs the risk of introducing an infection (Egg Yolk Peritonitis) or hormonal implants to prevent ovulation and therefore stop the condition getting any worse. The latter are expensive as they need veterinary insertion and require replacing every 3-6 months. The body seems to be able to reabsorb most of the fluid whilst on the implants but the egg yolks remain in the abdominal cavity as there is no way for them to exit other than by extremely risky and expensive surgery.
tumour .... is the third most likely option, which will usually cause the bird to waste away. Feeling the keel bone is usually a good means of assessing body condition. Sadly there is no treatment.
fatty liver haemorrhagic syndrome.... is also a possibility and is most likely to occur in hens that are less active (usually heavier breeds or birds that are penned in a small enclosure) and fed an inappropriate diet..... often too much carbohydrates like scratch, bread, pasta, rice etc. The bird will usually be carrying a good covering of flesh and fat. You will sometimes see Ascites (water belly) with this ailment too. Changing the diet of the bird and giving a vitamin B complex supplement to support the liver may help but usually the damage is done by the time you see symptoms.
Do any of the above options sound like they fit your birds situation? Unfortunately birds are very good at hiding illness until it is too late. Hens have been selectively bred by mankind to be finely tuned egg laying machines for the first couple of years of their lives and are then considered disposable. Their reproductive system cannot always handle this unnatural level of work and is therefore more prone to disease and malfunction.
I hope that gives you a little insight into the issues you may be dealing with and all I can suggest is that you make her as comfortable as possible and perhaps be prepared to euthanize her when quality of life becomes untenable.
I'm so sorry
