Isabella is a nice girl, cute and seemingly innocent besides her obvious attraction to Phineas. But those kinds of nice girls tend to have some kind of secrets going on.
Take Isabella for example, waking up late and getting dressed in a hurry meant that she missed 1 normally unseen piece of clothing that is quite important when waring a skirt or dress. Once she realised it was missing though it was too late to just run off and fix it without being suspicious. To walk around and act normal while being exclusively aware of how almost exposed you are. It's very exciting. So much so that eventually, she decided to try doing it again on purpose.
First two scenes are from her forgetting her underwear. Third is her taking them off... In public. Nobody saw her slip them off and hide them away, that would be too embarrassing. For now.
