See, this is an example in the purest form that everyone views life (or tv shows in this instance, lol) with different eyes. For me it was the pure shock from Carly that she never expected to view Sam as a rival for Freddie's affections.
She knew she could take top spot over Valerie or Rona Burger (fictional in iXMas) while she was figuring out thier relationship---really, who wants to find a soulmate at 13 before you've had time to play the field and live a normal life----but if Sam was to come into play, someone who she knows Freddie cares for, she wouldn't be able to sit back and watch it play out. She might miss her chance. I feel like that's why she freaked, because she had real compitition, she's always just kind of banked on Freddie being there-in love with her. It's very clear that it disturbs her to imagine Freddie not being in love with her. This was something that upset that balance of her descion, thats why I veiw it as more a Creddie than Seddie ep. Sure, it was about the Kiss, but it was mostly about how Carly reacted to it.