Chickens can get Mareks at any age, but it is more common in the first year of age. It can take 3 or more weeks to cause symptoms in a chicken that has been exposed. We cannot guess what she has though. If a chicken dies, it is best to get a necropsy if you want to pinpoint a cause. Some illnesses can be identified with some certainty with a do it yourself necropsy at home. If done right after death, coccidiosis may be seen before the intestines become darkened and grey. Take pictures of organs (liver, intestines, and as many other organs as you can.) post pictures here.