Knee replacements surgery is major surgery, and it takes many weeks and months for the tissues around the knee to recover from the operation.
In the early days after surgery your knee (and whole leg from knee to ankle) will swell, especially after increased activity.
For the majority of patients swelling will remain in the leg for weeks after the pain has gone. Symptoms of swelling alone is normal. You may notice swelling worse as the day goes on as gravity will keep water in the leg.