I'm just wondering about this as it happened again this morning.
DD is (just turned) 6 and for the last 2 years or so, if she's excited about something that's happening, she throws up. Just once, very suddenly - she literally bends over and gets sick, she says she can't make it to the bathroom because she doesn't know it's going to happen. She doesn't feel sick before or after. She's not running or jumping around when it happens, she's usually getting ready for school or something.
It happened before a teddy bear picnic at school and birthday parties that she was a guest at. It's not every time she's excited and it's never right before the event, it's usually a few hours before.
It's only happened about 5 times so I've never made a big deal of it. I usually just monitor her and if she's fine I send her to the party. Obviously if she was actually sick I'd keep her home but she's 100% herself.