Why does my knee hurt when I run?

I'm 13 and whenever I play sports my knee hurts. I'm an athletic boy I play football baseball and basketball. Whenever there's a game in the beginning my knee if fine them when I start to run it hurts bad. I tell my coaches and they just keep me out of the game. Why does it do this to me? Do I need to see a doctor?

Yes, definitely see a doctor especially if this has been going on for a while! (ie more than 2 weeks)
Also wow that is a lot of physical activity even for someone your age! You should feel proud of yourself that you can balance all that AND school, but remember to take care of yourself and your health, too.
Your knees are so vital & if you don't take care of them, if you mess them up while you're young they may never heal. :( So do see a dr. right away pleasd! :) I hope you feel better soon!!