Thank you for the additional information.
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 (which could possibly be palpated in the abdomen), a mass (ie. tumor) in their abdomen, ascites (fluid), peritonitis, or hemorrhage (which sounds less likely in her case). Just to note, any of these 'space occupying agents' will be extending her girth will affect her gait and make walking difficult and show the signs you are seeing. If we consider each of these potential causes in turn, you will have a better idea of what we may be facing with this abdominal distenstion.
First, we must consider trauma induced hemorrhage. This can appear as a bloated hen, but often we would also see paling of the comb, collapse, possible death. That said, a slow bleed could still be a consideration here.
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 fluidy 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. And if this one that other did have severe diarrhea recently, then this might be the underlying trigger for this. Because with diarrhea protein is lost, thus over a longer period (or profuse output), the blood protein balance will again be imbalanced.
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 her. 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.
No matter which differential is to blame, consider isolating this bird from the flock. .This bird should be moved to a hospitalization pen. It should be a small enclosure with soft flooring. Her food/water should be easily asccessable for her. This will remove any competiton and stress of being bullied, while it will allow you to closely monitor her (know if she is eating/drinking) and administer any supportive care measures. If she is struggling to eat and drink, you will know about it and be able to assist her.
As well, while we are monitoring and offering supportive care if she needs it, we do want her to see a vet. This would be prudent to do so as soon as possible since this mention of a warm belly (possibly fever) may support a brewing bacterial infection secondary to an egg peritonitis.
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, and quite importantly, they can check for fluid present in her abdomen and 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 and give you the best chance of getting her back to being herself. And if this is a diarrhea based protein imbalance, then vet will able to help you support her properly to resolve the abdominal fluid.
So, I would have her checked out by her vet. If you don't have a specialist avian vet, you can check where you can find one at near you at http://www.aav.org/search/, Avian web (LINK) or Birdsnway (LINK).
I hope this information is helpful.
Please let me know if you have any further questions.