Katniss and Peeta had not spoken since the closing ceremony that took place after the 74th hunger games had ended. During the ceremony they held hands and wore smiles for the cameras, straining to distinguish whatever tension resulted from the last conversation they had just shared.
Peeta avoided talking to Katniss out of the immense humiliation he felt upon finding out that Katniss, who had been the subject of his affections for so long, did not see him as a true contender for her love and her reciprocation of his affections had all been a mere hoax intended to ensure their survival. Katniss, in turn, evaded his gaze all throughout the evening out of the fear that her eyes would betray her and reveal anymore thoughts that could possibly hurt Peeta. She had already felt so guilty about what she had admitted to him earlier that day on the train—that all her kisses and attempts at affection had been a calculated effort to ensure their survival.
Her own mostly, as the intention to preserve Peeta's life in the games had been a mere afterthought, motivated by selfish needs. She had only expressed interest in protecting Peeta after it had been announced that both survivors from any district could win the games, and even then, her hesitation to spare Peeta's life had only surfaced in her mind after she envisioned the amount of backlash she would get from her own community for having killed a fellow district member and the shame that she would feel because of it. So of course, her actions had been entirely selfish, the whole way through, while Peeta's actions were founded in intentions that were so pure and selfless. How could she face those blue eyes when she knew in her heart that she deliberately exploited his feelings simply to reach her own ends?
In her defense, she was acting solely on survival instinct, and telling someone that you're madly in love with them whilst you're in a game of survival in which you're expected to rip each others' throats out and be suspect of any tricks to disarm you isn't the best idea. She tried to convince herself of this, but it just wasn't enough to push away the incredible amount of guilt that she felt. So, she did what she does best, and avoided any more confrontations with Peeta. Fortunately, she had the reunion with her family members to divert her attention from the constant stream of guilt that consumed her.