First, trauma induced hemorrhage can appear as a bloated belly hen and can be very quick in onset. That said, you would also expect you to also see a paling of the comb, collapse, possible death. That said, a slow bleed could still be a consideration, so check her comb/mouth color for paling of the mucous membranes.
Tumours in the abdomen are not uncommon, and are something we have to consider if one hen is affected. Often they will manifest as a hard mass in the abdomen but we can see fluids arising because of the tumor (since they tend to have abnormal
vasculature) Still, she is younger, so I would probably put this lower on my list initially.
If we then turn our attention to the pure fluid diseases, we have to look through the range of 'ascites' . There are transudates, which are most like ‘water’ on gross appearance. This is the fluid we tend to find with heart disease related conditions or those that arise will protein imbalance in the blood vessels. If the hen 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. (Often the fluid will have a yellow tint as well)
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.
As well, we can see bacterial infection cause signs of this nature, though the fluid they cause will be pus like and the bird often behaves quite poorly. This tends to arise when the bird has had a deep injury or any issue that would allow bacteria to get into the sterile abdomen. This turns into a bacterial free for all and the fluid that results will be pus (full of bacterial + inflammatory white blood cells) Finally, the worry we must always have in female birds, egg peritonitis. This is a very common abdominal ascites producing condition that we see specifically in the bird species. 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, when a bird has ascites these are the issues we have to consider and rule out. We do need to remember that despite her behaving quite normally, this is a huge red flat. As I am sure you will appreciate, birds are very good at 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, all too often they hide the early stages of disease from us and our only hint is the ascites and sometimes that is when the problem is too advanced to treat.
So, in Pepie's case, these are the differentials we need to consider. Each will have a different treatment option and prognosis associated with it. Therefore, the most straight forward way of determining which is the cause and what must be done would be to have your vet take a sample of this 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 Pepie back to being herself. As well, this will allow you to make sure this isn't something that is a concern for the other birds in your flock.
I hope this information is helpful. Please let me know if you have any further questions. If you have no further questions, I would be grateful if you would press the wee green accept.