How old is old? When was her last egg? Ever treated for internal parasites?
Have you felt for squishyness between her legs.. for EYP/waterbelly symptoms? How does the skin look below her vent?
Lungs filling with fluid *can* be a symptom of ascites (the fore mentioned waterbelly/congestive heart failure).
FWIW.. arthritis can have many causes including LONG term dietary
excess calcium or excess protein.. that effect kidneys and *can* cause gout, kidney failure, and well death.. usually in birds genetically predisposed somehow and ALSO fed an inappropriate diet for their age/gender.. Please describe feed routine. But the real part of information I want to share is that arthritis hurts always.. not just when it's cold, and not even just when you're moving around but even just sitting around can be uncomfortable (not the same as suffering or miserable)!
Regarding pain.. did you see any improvement in mobility after dosing with ibuprofen? I thought ibuprofen was against the rules for poultry but discovered that may not be true.. here is a thread that discusses proper dosing..
https://www.backyardchickens.com/threads/ibuprofen-for-controlling-pain-in-poultry.1250629/
Which I have no issue using pain meds once the issue has been identified and also if improving mobility isn't going to further increase the injury.. since sometimes pain and swelling are there to slow us down and protect us in that manner.
Hope she feels better and continues to thrive!