Ok.. she's not "sick" if she is pooing like that same as humans it's her body detoxing itself U need to give her healthy food clean water and clean the poo out of her coop or sleeping area if you are not doing it enough because they breath in the fumes from that and that can disrupt the terrain of the animal. The "medicine" you are giving her antibiotics ect.. from the vet is bad for her health it stops the detoxing process from taking place. Humans and animals do not get "sick" what most people think of as sickness is actually the natural detoxing of the body going on and needs to take place for the individual to get better, if you give her pharmaceutical drugs it just slows the process but issue will resurface again. Her feathery body is much clever then U think and knows how to regulate itself and health we just have to give the correct conditions for this to happen, which happens naturally when given the opportunity.
Also to add if another one of your chickens get "sick" like with humans they think they "catch" something from another it's not the case, it's either something in the environment or a symbiotic reaction the same way someone yawns and you "catch" it, if someone is detoxing near you "sick" and your body picks up on it, it can go ok.. there detoxing now i should do the same, if need be it is a reminder. The seasonal "flu" comes in winter it's a cycle of nature like summer ect.. like how some plants grow in different seasons its nature and just a natural process that will take place if it has too.