My total score on SO on C++ tag fell down from about 1100+ to 985 in last 12 hours. That incident changed my position from 16-th to 19-th place in C++ tag. At the same time my reputation wasn't changed. What's wrong with it?
My guess is that it was this post being made CW (along with all its answers). Although reputation gained before a post is made CW is generally retained, my guess is that that doesn't apply to votes for the stats pages.
Your answer on that post had 155 votes, so "losing" that answer would certainly affect your stats on the C++ tag significantly.
It sounds like a re-tag may have happened, where some of the questions were relieved of the
[C++] tag in favor of something else.
I'm not sure how to verify this hypothesis, but it explains your observations. Offhand I can't think of another hypothesis that fits.