@tennisgal Ouch! Setbacks after new PT exercises are not uncommon after TKR or any other surgery/injury.
Is this the first time the therapist had you do wall sits? Was it the registered PT, or a PTA (physical therapy assistant)? Were they at hand the whole time, watching your technique and level of fatigue? Did they limit your range for the squat to 45 degrees and make sure your feet were far enough from the wall? It sounds like this might be a case of "too much, too soon" or of poor form.
I would report this to both the surgeon and the therapist, ice and elevate, and continue range of motion exercises to the best of my ability.
And next time a new exercise is introduced, I would insist on doing half as many as the PT wants the first time, supervised directly by the PT (not PTA) to see how my body tolerates it.
If I got pushback on my requests, I would be looking for a new Physical Therapy facility - they are not all equal in quality.
I appreciate your kind response,thank you; to answer your questions,it was the first time I attempted wall sits,and was instructed to reach ninety degrees and hold for five count. After five or six I was struggling and told to take a break and continue to twenty. Interesting, I am not certain if it is a PT or a PTA and will now find out! I will follow your suggestions and advocate for myself,I appreciate your instruction, thank you.