What Causes Swelling In Ankles Of An Elderly Person?

my 91 yr old mother has been living with me for 6 months, she has Dementia. We just noticed her ankles are swollen. she does not have any serious health problems and only takes medication for Dementia and a vitamin. She is sitting most of the time since she has been with me. Too cold to go outside and walk. Maybe this swelling is due to sitting?? Thank you, Judy