Hello Amy,
We started off with ex-battery hens, they were brilliant birds, but unfortunately they are bred to lay a lot in a very short amount of time, not to live long lives.
You’re spot on on their care, maybe just take a pooh sample to the vet to check for worms (that should be really inexpensive, I remember paying £4 a few years ago). Most of the time chickens seem fine even though they need worming, but every now and then one bird really suffers under a heavy worm load even though the others can cope wit it.
Chickens can’t get fleas but they can get lice, tiny beige creatures, fast moving, that lay eggs in clusters on the shafts of the feathers - check the feathers around her vent, you may need to google a picture of lice eggs first.
Whilst checking her vent, clip away any soiled feathers in case she’s had an upset tummy, it makes it easier to tell if the upset tummy is continuing. You can use a normal pair of kitchen scissors for clipping feathers. She may have a bit of a bare bum but it’ll grow back quickly.
As mentioned above, what does her crop feel like and what is her food intake like? You may need to keep her in overnight, maybe in a utility or kitchen, in a crate or box on some bedding to be able to tell if she’s poohing and if so, if it’s not al pooh. One of the tell tale signs of a bird that doesn’t eat is if the pooh is bright green / yellow wet stuff - that’s stomach acids without food content. So if her pooh looks normal, she’s eating, if it’s thin and wet and bright green she’s not.
But there could be endless things wrong with her, and most vets have no idea about chickens, less than you will have. I had a vet say “the pooh looks good” whilst pointing at wet sludge, she should never have accepted that appointment in the first place.
Chickens are classed as exotics (yup...) so you may need to phone around for a vet that has experience with them. Chances are, though, that they’ll just give her a broad spectrum antibiotics, either injectable or oral. If you’re ok with needles, then they’ll show you how to do a subcut (subcutaneous, under the skin, easiest with 2 people) injection if it’s one of those that needs repeating at home. If it’s oral antibiotics, you can mix it with yoghurt, scrambled eggs, porridge or whatever, but you need to know first if she eats so again you may need to keep her indoors overnight and also check her crop whilst you have her there.
Do keep us up to date, you’ll find lots of help on here! Good luck!