My mom has developed a large black spot on the bottom of her foot?

My mom is type 1 diabetic and has very limited mobility due to not fully recovering from her last fracture in August. She had a rod placed in her tibia. She has developed a large black spot on the bottom of her foot and it’s very swollen all the way up to her knee. She saw her orthopedic Dr who gave her antibiotics. I’m concerned and not sure an orthopedic doctor was the best one to see. Should she get it checked by a different type of doctor? Can antibiotics fix this?

Age: 63
Female
Medications:
General Information: Diabetes, osteoporosis,. Gastroparesis, heart disease, high blood pressure. High blood pressure meds and insulin.