I'm Having Sharp Pain In My Right Knee, When I

I'm having sharp pain in my right knee, when I bend down on it (ex. like when I kneel beside the tub to give my daughter a bath). It's been going on for about a week now. It came on suddenly & I don't recall doing anything to cause it. It's been feeling worse today & began have pain (not as sharp though) when I bend my knee. What do you think it could be?