The back of my leg by the knee is warm, red and itchy and painful too. It's also a keloid scar from a bypass surgery 4 years ago. Is it infected?

Highly unusual. It is highly unusual for a wound to become infected 4 years after the initial procedure unless there are some foreign bodies such as permanent sutures or staples present. However, it is reasonable to see a doctor to help make this decision or to inject the scar with steroids to diminish the symptoms of an active keloid.
Cellulitis. Need to make sure about possibility of cellulitis. You may need antibiotics for management.