D.D.
I agree with Janet that heart issues can cause fluid relention big time. If her general doctor can't figure it out then she should ask him for a referal to a cardiologist who can look into it.
Also check her medications to see what side effects they have. My mom started on cholesterol medication and started having weakness. Her pcp diagnosed her with lyme disease and put her on antibiotics for a month. She ended up at a specialist and he took her off the cholesterol medication and her weakness went away.