Chicken sickness getting worse!! I need help!!!
Can you post some photos?
Sounds like a respiratory illness.
Any discharge from the eyes, wheezing, sneezing, coughing etc.?
I would separate her so you can monitor/treat her. A sick chicken sometimes will also become dehydrated so she may need a little help with that.
When you say she is extremely dirty, what is it from, her behind or is she laying in poo or does she have a "rotten" odor to her?