Yes, you were a little hot under the collar.

The previous sentence talked about anger.  Sometimes, when a person is angry, they get physically warm, and their face and neck can become a red color.  The ‘collar’ refers to the area by the neck, where the top button of a shirt is closed.

沒錯！你有一點「臉紅脖子粗」了！

人，生氣到某個程度，會有身體發燙，臉部及脖子都變紅的情況。句中 hot under the collar 這個片語，說明衣領 (collar) 以下的部位很燙。