This is the 5785th most frequent Japanese word.
にわか
Sudden, abrupt, or temporary; often implying superficial or unprepared.
Here, 'にわかに' means 'suddenly' and describes a quick, unexpected change or reaction.
その話を聞いて、彼はにわかに顔を赤らめた。
Upon hearing the story, he suddenly blushed.
In this sentence, 'にわかな' modifies '感情' (feelings) to indicate that the feelings arose unexpectedly or recently.
彼女に対するにわかな感情に戸惑っている。
He is puzzled by his sudden feelings towards her.
Here, 'にわかの' describes the support as superficial or lacking deep commitment.