[She might not really understand what's going on, but 'arriving' to this place in her Syaoran-inspired samurai outfit is indication enough to her that Something Is Up. She's read enough fantasy novels to know where this is going, after all]
[She can't exactly 'sense' the barrier, or anything like that, but she might have caught a glimpse of a monster a few hours back and that's enough to put her on guard no matter what. And so she's quickly running after Sansa, sword not drawn but at her side and ready. It's not her usual practice sword, but she'll have to make do]
Wait! You have to be careful! [So says the Rules of Fantasy. But you'd know nothing about that clearly!!1]