Why does my body shake uncontrollably during confrontational situations?

I work at Sainsburys and during work today I asked a guy for ID when he bought alcohol. He was already drunk and got very loud and aggressive because he didn't have any ID. He and his friends were shouting at me and I felt my body start shaking. It was lucky that the security guard was there, because he threw them all out. My colleague didn't say anything but I know he noticed my body shaking. It was so weird.

Has anybody else experienced this and do you know why it happens? Please don't be nasty.
Why does my body shake uncontrollably during confrontational situations?
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