I had the same issue.
knee slowly getting painful, weak, and eventually i couldnt even lift it to climb steps.
Considered the most likely source of transient bacteremia might be related to a bad tooth.
Took the appropriate antibiotic, and the pain that had been coming on slowly and progressively for 3 weeks went away overnight.
I kept at it for 3 weeks and then got a joint aspirate culture which was negative.
The doctor thinks I'm crazy to think I had a prosthetic infection, and I think he's wrong.
Look into bad dental bridge or partial denture situation in hubby and if suspicious treat for subclinical oral flora bacteremia, because it can happen without the fever and overt signs of sepsis.