It starts happening immediately after the announcement. She, uncontrollably, calls out Peeta's name followed by a million explanations. It's like she needs to rationalize her excitement over being able to partner with Peeta. Not that these aren't valid reasons but I think her shouting Peeta's name with abandon shocked her. She's used to being calculated and repeatedly she has reacted in unexpected ways when it comes to Peeta. This one puts her in danger though. She is trying to make sense of what she calls a stupid thing to do.

Once she realizes the rule change is because of the star-crossed lovers ploy she has a mixed reaction. She figures out Peeta's been trying to protect her all along and she feels something Real because it makes her smile, but she uses the smile to manipulate the audience.

Even after her rationalizing calling out his name she still doesn't really know why she came after him

As a teenager recognizing a crush for the first time, in the midst of a death match, how would you know what of your feelings is Real and not just trumped up from the situation? How would you know which actions you did because of Real feelings or because it was right to try saving them too or just in order to better your chances of survival?