Although my case is not nearly as complicated as yours, at week 4, like you my first 4 weeks were excellent I had followed BoneSmart recommendation and didn't do any physio until my 4th week, having had, the experience of my other knee replaced two years previously and I only went as the center is run by the hospital and a friend worked there. Unfortunately I was handed over to a young not very experienced physiotherapist whom pushed my knee to beyond its capabilities ( sitting on a bench with legs dangling ) the young chappy lifted my knee straight up to as high as it would go, as I was highly medicated (Tramadol, tylenol etc) I didn't feel any pain, that evening I tripped over my crutch, I didn't fall but felt very uncomfortable, the next morning, I was unable to put any weight at all on my knee, the pain was undescribable, I contacted the hospital and was told to ice, elevate etc, I did this for a week, it appeared to have improved but I noticed every now and then it buckled, (whilst away on vacation) a few days later it buckled on a staircase, fortunately I was holding onto the handrail, upon getting to the bottom of the staircase, I was unable to put any weight on my knee, nor was I able to bend it at all, I had gone from a ROM of 120 to 0.
It has now taken me 4 weeks of rest, ice & medication, I've been going to the hospital twice weekly for gentle CPM sessions, Thankfully this has worked and I've slowly regained my ROM and I am now able to walk with a crutch but still have trouble bending my knee whilst standing.
The moral to my experience is never go to physiotherapy whilst on medication, as you won't be able to say stop or no if it hurts.
@MarkTKR Had what happened to you on my 1st replacement, I too would be inclined to think that I wouldn't do it again, but I'm proof no two knees are alike, I sailed through my first replacement, two years ago & I now have a good steady pain free left knee. Thinking my current knee would be just as easy, I had my right knee replaced in December 2021, just over 3 months ago, I'm now positive that eventually it too will be good, but am realistic its going to take a long time before I'm able to walk normally. Stay positive as positivity wins this battle, ( I've been depressed but it did me no good) look at all the little achievements and celebrate them. With time you will get there xx