Actually, I had just this same issue two weeks ago. I had my left knee replaced in November, in January my doc said I could do almost anything if it doesn't hurt. But if it hurts, back off. Well, two weeks ago, started playing racquetball. LEFT knee felt great! Right knee had exactly the same pain you describe. In fact, it got bad enough I was certain at bedtime, I wasn't going to make it to work the next day. But, it felt OK but stiff when I got up. And through the day felt even better. In fact, played a slow game of racquetball that night. And no issue since. Had my first real run on the treadmill today. Felt good, but need my lungs back...