I have had that not so good feeling when doing the leg lifts. I find that if I am not absolutely perfect in my form and turn the knee to the side and keep it there (very important) and don't keep the foot flat out to the side and keep it there, I can get the same feeling.
I wouldn't think it is any kind of cartilage damage as much as a strain on the tendons and muscles of the leg surrounding the knee from both the thigh and the lower leg. After having a small meniscus tear treated with surgery (and chondromalacia of the patella trimmed off), I generally don't use the weighted bar (I only have an 8 lb bar) for the floor work (lying leg lifts) any more. If it feels more like you have a bone rubbing on bone (a grating, grinding or popping sensation), see the doctor. If it's more of a muscle ache, I'd rest up, ice it, and see how it does. If it continues on, though, head to your doctor.
If you haven't done the workout in a while, that will always kick in the extra ache. If you have been doing it, check your form and lighten the weight or eliminate it for now. I don't care for ankle weights much myself but placing them on the thigh is another way around it and moves the weights up past the knee and still puts resistance on the muscles. Hope this is of some help. Feel better!