Have you monitored her crop? It should be completely empty in the morning. A crop issue can cause watery stools and labored breathing.
I would also check her abdomen - cup your hand between her legs and feel back up towards her vent. Does it feel swollen or bloated at all? Compare to her sisters for accuracy.
Do you know if she has been laying? When was her last egg? Reproductive disorders can also cause labored breathing.
Photos of your chicken now, and her poo, would be really helpful!
I would also check her abdomen - cup your hand between her legs and feel back up towards her vent. Does it feel swollen or bloated at all? Compare to her sisters for accuracy.
Do you know if she has been laying? When was her last egg? Reproductive disorders can also cause labored breathing.
Photos of your chicken now, and her poo, would be really helpful!