What Causes Intermittent Cellulitis Post Knee Replacement Surgery?

my Mom had a knee replacement 2 years ago and has had celluitis off and on in her feet ever since. Do you think the knee replacement had anything to do with this? Her whole leg was purple after the surgery and some of it never cleared up. She has poor circulation and horrible, painful feet anyway, but still takes care of my Dad, who is getting to be a burden on her. He has dementia, but is not ready for a nursing home yet. Anyway, she went to her PC and he gave her Keflex. She Later went to a dermatologist and he gave her another antibiotic. Two weeks later, her feet are red, painful and still swollen.