。
She seemed flustered as she was reading the letter.
Literal
She [topic-は] that letter [object-を] is-reading [time-時] flustered state was.
A multi-clause sentence using ~時 ('when/while') to set the temporal frame: 'when [she was reading the letter].' うろたえる ('be flustered, lose composure, panic') describes a visible flailing — someone caught off-guard, briefly disoriented, perhaps fumbling for words or actions. ~様子だった ('had the look/state of') is the formal way to describe how someone appeared, often used in narration to report inferred mental states from observable cues. The whole reads like a witness's description of the moment.