I think its really a lack of sleep/rest. She's 16, and just 16 at that, such a critical age where you need to be sleeping at least 8 hours a night most nights and eating regular meals. I know Sulli is only 16 too, but she doesn't feel a sitcom every day like Krystal on top of all the promo stuff and CFs. + god knows how much Krystal works out... she loves exercise and her abs are insane so probably puts a lot of effort into working out away from practice as well. Then again, Jessica has passed out a few times too, but never on stage like that

Their father is an athlete, but stamina might not be something that is in their genes.
Its good that she lives at home with her parents though, we know she's been looked after that way.
My baby

Oh and Sulli picking her up bridal style 8D JungLi <3