I tried everything I could think of:
* "sore no more"
* foam roller
* a tennis ball
* rest (vs running/cycling)
* hot tub
That last item (nuun) came as a suggestion from a cyclist friend at work who felt that the knot was brought on by muscle cramping caused by dehydration/electrolyte imbalance. After reviewing my training/nutrition/hydration for the past few days, I decided he might be right. I believe that I do a decent job of hydration/nutrition when I'm cycling/running. But on gym/swim days - not so much. I distinctly remember commenting after my gym session with Trainer on Friday that I was sweating, yet I did nothing to replace fluids or electrolytes for some reason. Woke up the next morning with the quad issue. There could be a connection.
I've also ordered "the stick" because several of you use that and it seemed worth giving a try.
Anyway, at this point, the quad is feeling mighty fine. Not sure which of the thing(s) that I did are responsible, but for now I'm back to normal - whatever that it!