fatigue lowers when you are logged off or playing another class, which is why your glad fatigue went away then came back right away. It never reset just lowered because you where either not playing or playing another class.

As for weekly reset, it seems to be 1 week from when you first got SP on that for the week. Meaning if you leveled 1 class on thursday it will reset next thursday, if you leveled another class on friday it will reset next friday.

SE needs to give us like a progress bar or something showing how long until the next fatigue rank or something, so we can see how long it will be and can see how much it is lowering.