It is very normal not to be able to function properly for a day or two after a panic attack, and I certainly fall into that category.
Panic attacks expend a great deal of energy, as a lot of adrenaline is pumped through one's system. Even a normal person having a "fight or flight" response to a perceived threat, will require rest after such an episode. A panic attack is nothing more than this "fight or flight" response, but no actual physical threat exists.
As always, the very best to you, Jols!... Chuck