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?

Cellulitis: Need to make sure about possibility of cellulitis. You may need antibiotics for management.

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.