Almost sounds like heart failure to me. If you have a stethoscope and she will let you listen to her lungs. If her breathing sounds gurgly you can probably suspect it is her heart. I'm not certain what normal chicken heart sounds are supposed to sound like but in a human or other mammal, in heart failure the beat will be a galloping sound instead of a 'lubdub'.
I agree with Alaskan. It is heartbreaking for you, I know but your last act of love towards your RIR is to let her go peacefully if it is age so she does not suffer. Hopefully it isn't something that will go through the rest of your flock.
I agree with Alaskan. It is heartbreaking for you, I know but your last act of love towards your RIR is to let her go peacefully if it is age so she does not suffer. Hopefully it isn't something that will go through the rest of your flock.