I'm sorry to hear your daughter is going through a rough time. You are correct in assuming that the combination of having her wisedom teeth removed and then getting hit with the flu, has likely triggered an increase in symptoms. People with hyperadrenergic POTS seem to experience an increase in symptoms when any event occurs that causes an even greater release of norepinephrine. I am relatively newly diagnosed (Oct 08) and still trying to thoroughly understand the syndrome so I'm no expert but I've found on days when my symptoms are REALLY bad, it is best to stay very well hydrated, keep meals small but frequent, rest in a reclined position (not lying down....just makes matters worse when it's time to get up again), and no matter how exhausted, dizzy and sick I feel take several short walks...even if it's just two minutes at a time.
She needs to be reminded there will be better days ahead and to just live at her own pace right now.
I hope she's feeling better soon.
Take care.