Gill, I doubt this is related to the drugs, I had a spinal block, not a general anesthetic. I have had several major operations (significantly more major that a knee replacement) and are familiar with the aftereffects.
This is no more than a re-occurrence of the depression I have had for many years.
I'm fed up with TV, films, books etc. 5 weeks of relative inactivity has exhausted all my sedentary activities, I need to get up & out, doing stuff that I think worthwhile. Which i better do, rather than sitting in front of my laptop. 