。
She was flustered by what she had blurted out.
Literal
She [topic-は] herself [subject-が] ended-up-saying fact [at-に] became-perplexed.
~てしまう is the regret/completion auxiliary — adds a sense of 'ended up doing X (and now there's no taking it back).' 言ってしまった thus carries the nuance of an unintended slip rather than a deliberate statement. 困惑する ('be perplexed, bewildered, flustered') describes the helpless confusion that follows an awkward moment. The に here marks the trigger or cause of the emotion — 困惑する pairs with に for 'to be confused by/at,' a common pattern for emotion verbs.