My Mother Has Had One Leg Off Last Year Because

My mother has had one leg off last year because of ulcers caused by bad circulation.
She has now developed a large swelling on the one leg left, it looks like a bruise but looks different.
She is 88 years old and has just finished antibiotic treatment for cellulitis. I am concerned that this swelling may be the return of cancer or another ulceration starting up?
The swelling is brown but with a green edge before it returns to the correct colour skin.