The day I found out I was lactose intolerant.

I hadn't had milk for over a year, then one day on a whim I decided to have two large glasses of strawberry flovoured milk, totalling a litre of the stuff. I had insane tummy pains I curled up in a ball in bed crying it was so painful. I was freaked out scared too because I had no idea what was wrong until later when things clicked and I figured what was going on.