Is she in public school? Or well, any school really.
She's probably having a hard time sleeping, or calming down. Middle school is TORTURE. TORTURE. Theres so much pressure on you when it comes to performance its hard to deal with, but thats almost nothing compared to the survival skills required to just get through the day around other middle schoolers. Theyre cruel.
Once she started using that stuff to calm down her body becomes dependent on it. She knows it works, and its the simplest solution to her problem.
I'd say find her another outlet. Look closely at whats going on in school and home, with friends and boys. Some where there is something stressing her out and unsettling her. It could be something as simple as her diet, or not enough physical activity.
whatever it is i wouldnt panic. you've confronted her about it and gotten her answer, unless she's prone to lying I'd be inclined to believe her.