I did a low-effort ride (spinning bike, 100 watt avg. 45mins.) with a few unweighted squats after and then some stretching .... and it seem to have pretty much fixed it. Now my legs are just very tired but that's not really much of a surprise since I have likely been training too hard in the months since i started and have had the burning quads for a week now (that alone can't be healthy!). I still wish I had some medical explanation not pulled out of thin air though or at least some forum post from someone else who had the same happen. Oh well, I'll take it slow for some weeks and then only slowly ramp up after that.
Warning! Totally pulled-out-of-thin-air-googleitis-theory:
Having just read about 1) mitochondrial cells and how lactic acid threshold can be extremely low in some sedentary people and 2) excessive exercise causing mitochondrial functional impairment and 3) that some medicine can damage the mitochondrial cells (me having a history of both cancer treatment and many years of antidepressant medicine) this is the theory I'll stick to unless I hear otherwise from some smart people.
Morale of the story: Don't overdo it and stretch those muscles after a workout.