Relatively speaking, if you exercise a lot, it may cause injury, discomfort, pain or even swelling. At this time, mild cartilage damage can't be seen by MRI. So the situation mentioned in the question may occur, but we all think there should be no major problems in clinic, especially the MRI. Personally, I don't think too much should be considered. It's just that maybe too much exercise has been done recently, and too much exercise has stimulated cartilage. Then you can reduce the amount of exercise appropriately, and the rest may be relieved. Of course, if you still can't relieve yourself after rest, it is still recommended to go to the clinic of sports medicine or orthopedics to ask a professional doctor to see what is going on.
Another situation is that you haven't exercised since the operation, which may also cause this discomfort. Because there is something missing in the knee joint, you need proper exercise to maintain the knee joint in a healthy state. Such as moderate swimming, cycling, jogging and so on. Proper exercise can keep the muscles and ligaments around the knee joint in a normal state, which may be good for your knee joint health.