I am seeing lots of chatter and discussion around another Google update that started around Friday, November 16th.
It may be a Panda refresh or some other type of update. But the discussion seems to be implying there was a small Panda refresh this past Friday, November 16th.
If there was a Panda refresh, that would only put about two weeks between Panda #21 and this Panda refresh.
I have emailed Google for confirmation on this update and will update you all when I have word back.
Forum discussion at WebmasterWorld.
Update: See my updated post named Google Won't Confirm Weekend Update But Says Panda Coming Soon.