Knee Problems

Up until recently, I've never had any problems with either of my knees. Last Friday, pain that had been steadily increasing in my left knee for a week and a half culminated in a sleepless night of painful twisting and turning, and I ended up limping and not being able to straighten in completely without a lot of pain the next day. The doctor suggested I take 1200 mg of Advil, which I did, and also sent me for an x-ray, which I also did, but when I woke up Saturday, all of the pain was completely gone and hasn't reappeared since.

Now, I'm guessing one of you has seen or experienced something like this before, so my question is: is this just a freak occurrence, or should I take special care of my knee, lest it happen again without warning?