I've been noticing this pressure in my knees. The pressure sort of feels like something is building up inside. It's random and just comes and goes. I'm fifteen, pretty active, female and consider myself of average fitness.
I would recommend you to visit an orthopedic soon. Taking risks with your knee is not advisable as it maybe something serious like Patellofemoral knee pain. Instead of aggravating the pain a doctor should be immediately consulted.