Interesting Sarah... I also have one of those stories, so if you've just eaten you should ignore this one too.
One time I had a fairly bad stomach upset for about two days. All I did, other than walking to the doctor, was lay on the lounge and attempt to eat occasionally. I lost 3kg in 2 days
. This was no good for me at the time because I was 62kg (my goal at the time) and it dropped me to 59 which is almost too low for me according to my BMI.
Anyway, my point is that the amount of food in your body at any given time can have a big impact on your weight.
By the way, I'm totally not advocating vomiting as a good way to lose weight, as well as being bad for your health it's just a yucky way of doing things.