I agree. If you read this
Energy drain for TKRs you will see that it lasts at least 3 months for just ONE knee. And you had not only two knees done but one was a revision to boot! So I would suggest that in your case, the energy drain
and the general healing process. Plus you had your first day back at work. You said it was an 'easy day' - did you forget everything that goes along with going back to work? I'm referring to all this - it all adds to the burden
~ having to get up earlier in the morning
~ getting washed/showered, putting on the 'face',
~ getting dressed
~ getting a fast breakfast and eating it in about 5 mins instead of casually
~ getting out the car
~ driving/travelling to work
~ walking from car/transport to office
~ actually being up and about and unable to take intermittent proper rests during the day
~ walking from office back to car/transport
~ driving home/on transport
~ putting car away/walking from bus stop to home
~ making an evening meal
~ probably feeding pets and/or others
~ washing up and then finding it's almost time for bed!