You only had your TKR in mid-January, so it's only been 2 months. That really is a very short period of this year-long recovery, and it's too soon to start thinking that something is wrong. And there is your answer! Your muscles were weaker and they have a lot more recovering to do before they regain full strength. Meanwhile, they aren't supporting your knee properly, so it feels unstable. If your physio is any good, knowing your past history, he will make allowances for it and stop telling you that what's happening is not normal. It is! Give your knee a little longer, so that your muscles are stronger, and you could well feel that your knee is more stable. Just don't try to rush things. Meanwhile, keep using the crutches, so you don't fall. Your knee will tell you when it is ready for you to give them up. Don't compare your knee and your progress with your mother's TKR journey. No two knees are alike and no two recoveries are alike. For all you both know, your knee could have needed much more work during your surgery. I bet your mother didn't go into surgery with muscles already weakened by waiting 6 years for the surgery, either.