Knee problem

Four and a half years ago I underwent an anterior cruciate ligament replacement operation. Last year I had a piece of bone that was floating around removed from the same knee! Having had physio, I returned to training – weights and JiuJitsu. Just recently my knee has swollen slightly and ‘clicked’ on full extension. There has been no pain and no real loss of movement – apart from that due to the swelling, which is relieved by ice! Can anyone tell me if my knee is just protesting at the raised intensity of activity and will get used to it, or if I am damaging it again. Perhaps it’s just a question of getting some ultra-sound to break up some free-floating calcifications? I am going skiing in April and could do with some advice on achieving a decent level of readiness!
Thank you and sorry for the long-winded question!