No cartilage in knee

I had keyhole surgery less than a year ago on my left knee because I had no cartilage left in it. I had been playing squash previously for a long time. After the surgery I was on crutches for a couple of days and it was awhile before I could go back to playing squash. When I went back, my knee was ok, it was painful a little bit but not too bad. Lately it has been really painful so I was forced to give up squash and then my knee was fine with no pain. But the past 3 weeks, it has been very painful again. I cannot understand this, as I am not playing squash or doing any impact sports anymore, yet it is hurting worse then before. Has anybody else experienced this type of thing? Any advice?
Thankyou.