Activity related? Did you injure your knee or is this a chronic problem?
Many causes. If you had an injury please seek immediate medical care. If pain is chronic and insidious in onset, it could be a variety of causes, but inflammation, arthritis, patellofemoral pain syndrome, meniscus tears, etc are some of the causes. Would need more information to answer it more specifically. Best to see an orthopedist about it and have it worked up.