My mom's first knee tka was 3 weeks ago. all fine except problems regulating body temp esp at night. no fever. post-surgical disruption ?

Possibly: Older patients can definitely experience a variety of disruption of body functions after a hospitalization or surgery. With any chills or change in temperature in an elderly person, it is important to rule out infection, though, especially after a joint surgery. I would track her temperature and talk to her doctor, who may want to check her blood and urine for signs of infection.