Yes, you really do need to understand that.
I don't know if you're squeamish, I am and couldn't even make it through the animated version, but if you can handle watching what's done during the surgery on Youtube, I think you'd have a better understanding of the pain we have as we s-l-o-w-l-y heal.
Ask your surgeon if he will confirm that the knee can be manipulated rather aggressively during the dislocation process, resulting in some discomfort / pain lasting weeks to months post op. It can also feel bothersome as result of your changing gait. Remember, our gait changes as our joint deteriorates, then changes once again after surgery as our body adjusts to the prosthetic. There's a whole lot going on there underneath the surface as your body is doing what it is supposed to be doing s-l-o-w-l-y healing.
Keep smiling and carry on, it's all going to be ok.