I am very concerned about your hen. When we see a chicken bloated we have to consider a few different reasons for this. First, we have to consider the nature of what is 'filling' her abdomen. We can see bloat in hen's that have a a bound egg, a mass (ie. tumor) in their abdomen, ascites (fluid), peritonitis, or hemorrhage (which sounds less likely in her case). Just to note, any of the 'space occupying agents' will be compressing the gut, so this may be why she has gone off her food and may be having fewer (and possible when she does defecate, larger) bowel movements. As well, any of these extending her girth will affect her gait and make walking difficult.
Trauma induced hemorrhage can appear as a bloated hen, but I would expect you to also see a paling of the comb, collapse, possible death. That said, a slow bleed could still be a consideration if we are seeing a paling of the crest.
Tumours in the abdomen are not uncommon, and are something we have to consider if one hen is affected. Still, she is younger, so I would probably put this lower on my list initially.
Ascites is the accumulation of fluid in the abdomen and is a very possible cause for the bloating you are seeing. We can see ascites appear in chickens for a number of reasons. If she has a heart condition, the mismatched pressures in the circulation will cause a back pressure and fluid will leak from her blood vessels into her body cavity.
As well, if she has liver disease (which you might also notice a yellowing tint of her skin/comb/mucous membranes) can cause ascites since a suffering liver won't be able to produce albumin (a blood protein it makes to regulate osmotic pressure in the vessels) and thus cause an imbalance in the blood's protein levels. A similar problem can also be seen if the hen has had a long history of diarrhea. Because with diarrhea protein is lost, thus over a longer period (or profuse output), the blood protein balance will again be imbalanced. And we must consider, that some tumors can cause ascites, as well as take up space, by their mucking with the body's blood supply.
Now in laying hens, we also have to consider those troubles that arise from the reproductive tract. Egg binding is a big issue that can cause a bit of bloating and decreased fecal output, though we also often see straining and such. As well, there is the issue with egg peritonitis. Because the hen's ovary and reproductive tract aren't actually connected (imagine the ovary throwing the ovum into a basket ball hoop to make a successful egg), a stressed hen can occasionally have their ovum not make it into the tract. When this happens the material ends up free floating in the abdomen. This is problematic, as I am sure you know, because 1) it doesn't belong there and inflames the delicate tissues, 2) it is the perfect media for growing bacteria. So, if a hen ends up with an egg peritonitis, we can see bloating, lethargy, anorexia, and it can progress to a fatal state.
So, these are what we have to consider with your wee hen And the problem I am sure you will appreciate with birds to that they do a very good job of covering up when they are sick. This is because as a prey species, attention to your illness will make you a target for predation. So, if we are seeing this bloating, then this might be our only hint until the problem is too advanced to treat. Therefore, we must take this as a serious issue that needs to be resolved.
I would advise moving her to a little 'hospital cage'. Make sure she has access to food and water near by her. You can place a heat pad under half the cage to give her the option of warmth. Or you can use a clean sock, and fill it 2/3 with uncooked white rice. Tie it closed and microwave (approx 1-1.5 min). Make sure to shake it before adding it to the cage, to allow the heat to distribute. Make sure its not too hot (as we don’t want to burn the bird. If it cools, you can re-warm as required). This is quite a good way to keep them warm in their carriers en route to the vet as well.
Since she isn’t eating/drinking, then you do need to consider intervening at this stage. You will want to monitor her water intake, since as I am sure you can appreciate, dehydration can weaken a bird and contribute to worsening her illness and cause additional issues. To maintain her hydration, in a drinking bird, you can offer water with electrolytes instead of plain water. There are readily available electrolyte solutions available on the market (ie. Vi-tal) or you can use Pedialyte or diluted Gatorade (diluted 50/50 with water).
You can offer these in a bowl or if she isn’t drinking on her own you can administer fluids (and hand feeding) via towel restraining and a syringe or dropper. Wrap bird “burrito style” and hold securely upright in lap. You can drip water on top of the beak, as reflex will cause them to catch the droplets with their tongue. (some will even drink from the syringe directly). In doing this, do make sure not to get water into the nares.
Feeding wise, we want to make sure she keeps eating. If her appetite is waning, try her favorite foods. You can also get Nutrical paste to supplement her diet (either mixed in food, water, or via syringe) which will provide extra calories or nutrition. Offer fresh foods, high in nutrition and water content like cucumbers, Romaine, grapes, melon, oranges, etc. Hard boiled eggs mashed shell and all are extremely nutritious and delicious to birds. And cooked brown rice is good for them too.
While we are doing all this, we do want her to see a vet. The vet will be able to have a feel of her abdomen, perhaps xray her, and determine the culprit for this bloated appearance. As well, if it is fluid that is present in her abdomen, the vet will be able to take a sample of the fluid with a sterile needle and evaluate it under the microscope (as a lot of the ascites have distinct appearances) Depending on their findings, the vet will to provide appropriate treatment, give you an idea of her overall prognosis and give you the best chance of getting her back to being herself.
If you don’t already have a specialist avian vet, you can check where you can find one by using the bird filter on the RCVS register to find a bird vet near you. You can find that here (LINK). As well, you can find a few more avian vets listed via the Avian Web (http://www.avianweb.com/recommendedvets.htm#UK)
I hope this information is helpful. Please do let me know if you have any further questions. If you have no further questions, feedback is always appreciated.
All the best,