Does she say it as a question or as a command or exclamation? The only interpretation I can think of is the same as
@Dentanmono 's. Ah So? Can mean "Oh really?" or Ah So! can mean "I see." Nothing particularly aggressive about it in general. Now if she says this and then kicks the cat, then we can come to the conclusion she's using it in an aggressive fashion.
OR if she is saying Uso! then that means "no way!" or "I don't believe it!" Like if the cat just puked a hairball in her shoe she might say "uso!" to mean "w t f not again!"