I don't recommend antibiotics willy nilly but you may want to consider talking to a vet about it.
This symptom checker, whilst not completely comprehensive has some good pointers to look for as well
https://cluckin.net/the-sick-chicken-symptom-checker.html
that would sort out gapeworm I think.
Could it be the heat? Have they got sufficient water, my hens like to paddle in a shallow bowl to cool themselves down. I also put the sprinkler on to cool the air around them sometimes.
Check her all over for a hidden injury, especially a small puncture...