When i look back I think mine was a combo of two events that happened close together.
One was I myself got sick at school. First on the bus, then again in the hallway. I was 8 years old. That morning I had told my mom I didn't feel well and she said I was fine. I wasn't fine.
Then my brother got sick in the back seat of the car on the way home from the grocery store. I remember plugging my ears and crying the whole way home. I assume by this event I was already somewhat emet based on my reaction.

I was a very shy kid. I think being sick in public really traumatized me. I can name EVERY single kid from 1st grade to senior year who ever got sick in class.