'grow flustered' is correct and usable in written English.
It is used to describe someone who is becoming increasingly flustered or frustrated. For example, "When the teacher asked her to explain the math problem, the student began to grow flustered".
"I wasn't thinking about trying to get some alibi for anyone," Sam said, growing flustered.
At one point he stared so intently at his co-anchor, Savannah Guthrie, that she grew flustered.
Californians waited in line for hours for the "horrible coffee," while Starbucks grew flustered at the use of its "protected trademark".
Members of the audience said later that Mr. Clinton grew flustered when he was asked during a question-and-answer period about the pardon of Mr. Rich.
Abbott grew flustered with the heightened surveillance, and walked a batter, bringing up Mejia, who quickly fell behind, oh and two, before lacing the third pitch deep into center for extra bases.
When HuffPost asked her what caused the increase, she grew flustered and said she would check her datebook and send back an explanation.
After growing flustered at the nature of the interview questions, Palin clarified on Monday that she holds Obama responsible for the "level of disrespect for the United States military that is made manifest in cutting budgets and... not letting our military doing the job that they're trained to do".
Everyone who has seen Pennington stresses how fundamentally sound he is: his footwork and balance are good; he makes correct reads; he does not get flustered, perhaps the product of growing up with a football coach for a father.
The Slows had grown used to harsh treatment, and when they encountered one of us who treated them courteously they tended to get flustered.
"He doesn't get flustered.
