A few minutes later ...
After Marc's update:
Isn't this because your available flags vary with your flag weight?
Perhaps it changed and now your total flags are less, but you already spent the equivalent for the old value for the day.
Of course this is speculation that'd need to be confirmed by a dev.
Update: I can confirm that the reverse is true -- I ran out of flags on SO and after my flag weight increased, I had more to use.
Closing as too localized; I expect this relates to:
Yesterday, at around 6:30 UTC, the flag weight algorithm was tweaked, and existing flag-weights refactored. You might have had 740 before-had, and 620 afterwards. The number of available flags depends (in part) on your flag-weight, hence this will have reduced.
And if you'd already spent all the flags available, you could well have spent more than you now theoretically have.
This only impacts yesterday, so I'm not overly concerned... and the UI is, in many ways, entirely correct.
Sigh; ok, to make things simpler, I've changed things to just display 0 if you've gone -ve.