Knee troubles

I'm having ligament reconstruction on Tuesday
For the past few days if my knee is bent for a few minutes and I straighten it I can feel something pop under my knee
I was just feeling and its right where it bends on the back of my calf and it feels hard and swollen
I'm gonna see if my dr can look at it tomorrow while I'm there for pre-op lab work

Anyways if anyone has had this happen what was wrong?