What Causes Numbness In Knee Caps?

out of no where for the past probably two weeks my knee caps have been going numb, usually when i am sitting. But even after i get up and walk around for long periods of time they are still numb to where i can pintch them as hard as i can and it wont hurt. I am a 15 year old girl almost 5 feet 7 inches tall, & in the past i have hurt both of my knees, but not EXTREMLY bad.