Being fatigued during the flu is very normal as well as being fatigued after the flu is normal too.
Usually when the flu has gone away the fatigue can linger for as long as 4 to 5 days although in most cases the fatigue only lasts a couple of days.
If the fatigue lasts longer than a week after the flu then you still might have the flu or a more severe strain of the flu.
In my experience when I get the flu I'm usually fatigued still for a couple days after I recover from the flu and I'm usually full of energy again after the few days have passed.