Posie and Della

Posie (Full name: Posie Sessedria Byke) is from a world where everyone has a demon side from birth. They use biters to stay calm and prevent the demons from taking control of their bodies. Her demon is Della Monica. After Posie's boyfriend cheated on her, she started acting strange, almost as if she wasn't even herself. Did Della Monica take control? Or is Posie just being strange just because?