Thanks for the input, guys.
I'm about to go to the MRI scan this week - both knees.
I'm doing bench & shoulder presses + some assistance exercises twice a week, while anything involving leg muscles & joints (i.e. light stationary bike) provokes something of a reaction in my legs - not pleasant, so I hope the MRI scan will show what's going on in there. The doc suspects I might have a meniscus rupture.
And yes - one wouldn't believe how much dystrophy of leg muscles can happen in a rather short period of time.