What Causes Sharp Pain In The Center Of Both The Knees?

I have a sharp pain in the center of both of my knees (under the knee cap) every time I bend my legs. I'm an athletic 15-year-old, and I used to have Osgood Schlatter's disease. I went to my Doctor to see what the problem was now, but she never told me what it was. What is it and will this go away?