Hello, I'm 15-year old girl. Two days ago I did some sports like running and playing basketball. Since then, I feel pain in left my knee and upper thigh. I can walk, but if I sit down and try to extend my legs or lift them, I feel immense pain (like pulling). Also, when I try to put even a little weight on my bended leg, I feel sudden sharp pain. What is this? and how could I treat it?