What Causes Pain Below Knee Cap?

im 21 years old, 5'6'', about 125 lb. a little bit below my left knee cap, it hurts when i put pressure on it. it doesn't hurt when i bend it or stand or walk but it only hurts when i do a squat so to speak. pretty much it hurts when i put pressure on it so i can't run that much. what is the treatment in order for my knee to get better