Weird!
Isabelle knew something was really weird when Ava Cassia suddenly screamed. First of all, Ava Cassia wasn't afraid of bugs or insects. She was a warrior princess with lots of outdoor experiences, scouring mountains and rivers to break through her <Aura Mastery>.
And second of all, Ava Cassia would never scream like that, whatever the reason. She was a dignified, stoic, and refined person through and through. Ava Cassia would never be caught off guard.
The scream was very out of character for her, so there must've been a deeper meaning behind it. Isabella just couldn't let go of this amiss feeling after seeing it with her own eyes in today's event.
Not to mention, as her distant cousin, Isabelle felt some kind of a different vibe with today's Ava Cassia.