Jeepers! How often do you rest man?
Kidding aside, I guess it is because of her low CON. However, being fatigue is a minor problem. I tend to rest first whenever I enter a new area anyway.
Here's a tip, give Aerie the Girdle of Fortitude. With the proper setup and spell list, Aerie can even become a powerful melee combatant, but I usually use her as a background spell-caster.
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If not a Sorcerer, then a Paladin.
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Baldur's Gate Item Mart