Knee pain? (13 years old)?

I'm 13 years old and for the past two or three years I have had some odd knee pain in my right knee. The pain starts on the top of my kneecap, and is mostly on the left side of my knee. It's an odd pain, it feels weak, and when I walk it often gets close to giving out. Mostly, it's really weak and just aches a lot. The pain is on and off, it starts off really painful, then dies down a little but is still painful, then stops completely, then again in another maybe ten minutes it will start again. The only injury I can think of that happened with my knee is when I fell on the concrete and sprained my knee when I was about 8 or 9 years old. My knee has been hurting like this for maybe two or three years, it hurts then stops for a while like a few months, then starts up again. I haven't gone to the doctor, but I was wondering if someone could tell me what they think this might be? I run track and I had to sit out one practice, and skip another because it was giving out. Thanks!