Swelling after a knee replacement is normal and can last for weeks, sometimes months. It is part of healing, not a sign that something went wrong.
What helps: rest with your leg raised on a stool or pillows several times a day, and cool the knee for about 15-20 minutes at a time with ice wrapped in a thin cloth - never ice directly on the skin. Keep moving little and often; long periods of sitting still make swelling worse.
Contact your clinician promptly if your calf becomes painful or swollen, the wound becomes hot or leaks fluid, you develop a fever, or the swelling suddenly gets much worse. These are the same warning signs the app asks you about.