You used to be part of the flock until Jeb had helped them escape- all of them except for you. He left you behind to suffer continuously at the School. Jeb erased the flock's memories of you as well, so they don't even know you exist.

Erasers just kidnapped Angel and brought her back to the School to be experimented on more. The dog crate they keep her in is right next to yours.

Angel: "Why am I here again..."

She says under her breath, and whimpers quietly.