It could be just that your chicken is starting to moult and is being bullied though it doesn't sound just unhappy but rather ill.
First thing I'd do is check her for anything obvious like egg binding, or impacted crop or sour crop. The next thing is good nursing, isolate her and keep her as quiet, warm and unstressed as possible. Keep her under observation and If she continues to show signs of depression, or deteriorates you should consider a visit to the vet. Things to look out for are droopy wings, loss of appetite, runny nose, sneezing, diarrhoea, matted eyes, discharge from the eyes, pale shrivelled comb or darkening comb. Feed her on a staple diet of layers compond feed with a little mixed corn in the evening and cut out any treats or other suppliments for the time being other than possibly a multivitamin in her water. Don't give her a cocktail of things "which will do her good" like extra protein (cat food), cod liver oil, yogurt, honey, poultry spice, extra calcium etc. Keep it simple good nutrition and plenty of fresh water. Make sure she's drinking well. If she has diarrhoea you might consider an electroyte in her water so she doesn't become dehydrated.
A healthy chicken should be active, inquisitive, bright eyed, alert, foraging for food, eats voraciously and drinks regularly, preens, lays and interacts with the other chx.
Personally I wouldn't mix barely acclimatised exbatts with pure breeds as they can cross infect each other. Exbatts can be carriers for diseases they have been vaccinated for and pure breeds are often not vaccinated and can expose the batts to infections their immune systems haven't built resistance to yet.
Stress is often a trigger for latent disease in chx and it could be that the stress of more dominant chx bullying her has caused some illness to develop.
Regards