Popping sound but no pain in my knee

Dear Ask The Doctor: Yesterday i squated down while working and i heard a poping sound but no pain in my knee. I stood up and felt just fine and went back to work. Well later i went to squat down abut this time i couldn't. the pain was like someone stuck a knife in my knee in the front part and i cann't bend down now. What might it be?

Dear Chad:

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