85 yrs old and pretty healthy, than after diarrhea, malnutrition, why swollen legs?

My aunt lived alone, and got diarehea, for a couple months, went in the hospital for 9 days, nursing home for 3 months. Now in isted living, back to her old self again, except her foot and leg are swollen. They have her on water pills, but she has a bad bladder too, so she is miserabley urninating constantly. Why would her legs be swollen, and the doctor says it happens in old age, and hasn't tested her for any other causes.