Hello There.
I can understand your concern.
I'm pretty sure you have not consulted a doctor and are just saying that no one is able to find the cause.
its not difficult to find the cause.
take her to a MD consultant physician or a general surgeon . he will evaluate her completely and order a few tests, get those done and he will be able to tell the cause.
Heart diseases, Blood pressure,
Kidney disorders,
Thyroid and other hormonal disorders, DVT, Cellulitis etc are a few causes for such issue.
Best regards,
Dr. Anil kumar jain.