Because I had an infection which required a second surgery one month post op, I did NO exercises for a month and was in an immbolizer 24/7. No bending for a month. After the month my OS prescribed non-aggressive PT. When I asked him what that was, he gently moved my right foot about 2 inches with his foot. That type of therapy went on for another month, all that was added was pedaling on a recumbent bike. My OS said that my soft tissues didn't react to aggressive therapy at all. I did have the rubber band feeling the first month before the infection/second surgery. But what helped my rubber band feeling was doing nothing. My TKR was straight for a month and even walking was difficult. Once the immbolizer came off I just walked around the house trying to bend and doing the heel slides as my OS said.
I went back and read that Jo suggested chronic pain syndrome. Were you ever able to contact your neurologist? Also, even though water walking feels good, it provides natural resistance (think weights) which might aggravate that tight feeling. The best thing I did was get on a recumbent bike. At first I could only rock the pedals back/forth, but eventually I got a full revolution with 0 tension. Then I'd pedal every day for 5 minutes and after a week or so 10 minutes. It really helped to loosen up my knee. Three years later I now do 8 miles in 45 minutes.
I wouldn't suggest not bending for a month, but the lack of aggressive therapy helped my tight band feeling. Also when the immbolizer came off, my knee wasn't swollen at all, which helped with the bending, even though it felt stiff.