Also a very good test is to feel her chest , if you can feel her chest bone easily ( I.e ver obvious like there is hardly any muscle) , this means that she has been sick for quite a long time and if she is showing signs of symptoms she is extraordinarily sick. When a chicken shows that she is sick she is very sick because as they are prey animals . Due to this they hide their sicknesses because if a fox were looking at a flock of chickens it would go for the unhealthiest one . So if a chicken is showing sickness she can no longer hold the poker face.
Another test to see if the chicken is very sick is to see if the dominant chicken in the flock was picking on that chicken because back to the fox reference , other chickens don't want to be around a poor helpless fox prone attack chicken.
Finally the last test would be the chickens cone , if it is not standing up straight or is slightly discoloured there is something seriously wrong .
Due to her symptoms that she doesn't want to eat and she smells really bad unfortunately I would diagnose that she has egg peritonitis ( every flock owners worst fear). 5 months ago one of my chickens presented these symptoms which it seemed to be egg bound so we took it to the vet because we could feel a egg , on the examination the vet also thought it was a egg until the vet started to pull out cheesy stinky infectious material ( chickens don't make puss as a infection . They produce a smelly cheesy hard substance . ) unfortunately she passed during the surgery. Yesterday one of my chickens also passed which was devastating because she was fine the night before and then she was laying on the coup floor in the morning breathing really heavily and not moving . We took her to the vet and unfortunately the best humane thing was to put her down
I am really sorry for your hen. Let me know. About her chest , cone , social status and breed of chicken and I can roughly determine if it is egg peritonitis but for absolute results you would have to go to a vet .
