You never want to use antibiotics on chickens unless you have a diagnosis or know what you are treating. Clean up the vent area with a butt bath if the chicken is not too weak and unstable. Look carefully for any irritation, prolapses, etc. Is the chicken lethargic, eating or drinking. Poop pictures can help. What does the crop feel like—empty and flat, full, hard, doughy, or puffy. Is it emptying by the next morning? Can you try giving some electrolytes with vitamins and get her drinking. Have you ever wormed your chickens? How old are the chickens that are sick or have died? Most state vets will perform a necropsy (autopsy.) That is the best way to know if you have a disease in your flock.