Five weeks is not enough to fully recover from a knee replacement
. It takes months of diligent, daily exercises
to regain full use of the knee. If you quit too soon, things can go backward pretty quickly, as you're finding out.
Insist on getting back to see a therapist, or find a way to continue to do the PT on your own, at least twice a day. Keep at it even if it hurts - knees take a long time for the complex structures to heal. Get on your feet and stay as active as possible.