I had a similar experience about 7 weeks after the same surgery. I was playing tennis, and I felt my knee hyperextend. I later developed chondromalacia patella, and my OS thinks maybe that event kind of tweaked things and caused inflammation behind the kneecap. I'm doing OK, but chondromalacia patella doesn't go away, so the knee isn't 100%
My advice would be to make sure you're keeping up with a program of physical therapy to make sure your kneecap is tracking correctly and to strengthen your muscles. The strengthening of the VMO and stretching the ITband has made things improve a lot.
Good luck!
Cindy