
Welcome to BYC and I’m sorry you’re having a worrisome problem so soon.
I’d change your title to “chick with diarrhea” to get more advice on your problem.
Having said that, Its possible that your chick has coccidiosis. Cocci is an organism that is in the environment but can overwhelm a fragile chicks gut quickly.
This is potentially life threatening and should be treated aggressively.
Testing is possible, (but not necessary) by a local vet, even if they don’t treat chickens. Many will run a fecal sample for you just like they do for dogs and cats, since the organism can be seen under a microscope. The testing generally runs around $20. Generally you go ahead and treat based on the symptoms.
Have you been feeding your chicks a “medicated chick starter” or no?
Typically the additive in the chick starter, a medicine called amprollium, is enough to keep the organism load below a certain threshold, but it won’t cure a case of coccidiosis.
Diarrhea that’s worsening or bloody stools, and lethargy are the most common symptoms.
CORID is the medication you need to treat her and the rest of your chicks ASAP. You can buy it at your local feed store.
Be sure to provide fresh water and keep your brooder extra clean since the chick is shedding cocci in her excrement and exposing the other chicks to it.