Knee problemsIm 15, and am very active. I work out and bike about 10 miles...

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...everyday, but for the last month o? Im 15, and am very active. I work out and bike about 10 miles everyday, but for the last month or so ive been having problems with both knees. Whenever I try to run I lose my balance and fall, and I have a terrible grinding feeling in both knees when i walk. So i was wondering what might cause this, and how to get rid of the problem. Thanks