如果您想说突然不快乐了，可以说"I suddenly became unhappy"。这个表达来源于英语中"unhappy"这个单词，意思是"不快乐的"。
例句：
I was feepng great, but then I suddenly became unhappy.
我感觉很好，但后来我突然不快乐了。
She seemed happy at first, but then she suddenly became unhappy.
她一开始看起来很快乐，但后来突然不快乐了。
He was laughing and joking, but then he suddenly became unhappy.
他在笑和开玩笑，但后来突然不快乐了。