What Does Recurring Popping Knee With Moving Knee Cap Suggest?

Hi I am a volleyball player that had a partial meniscus tear. I had surgery and it has been fine ever since. Probably since I was about 13 I started having knew experiences. What I mean is I would be sitting with my leg bent under me and I would turn to stand up and I would feel something f move out of place. I learned over time that I had to straighten my leg fully to pop it back into place. When I staighten it it pops and my knee cap moves back into place. It has been happening more and more often now. What could it be?