Studies show that strength deficits can persist months to years after a total knee replacement. And even when strength is normal, movement patterns learned when you were in pain don't go away without retraining.
You may be in this second category. Patterns of movement adopted to avoid use of the quadriceps muscle before the knee replacement may still be present. A physical therapist can help you change the way you move.
Compensation patterns used to avoid pain can actually place extra load on the other leg. The long-term result can be arthritis on that side. The therapist will test your muscle strength to be sure this isn't part of the problem. Functional tests of speed and endurance can help identify the specific problems to be addressed.