It would be from someting you ate within the past day...nothing from last week. If you posted everything you ate we could probably pinpoint it. Just let it be reassuring that your DS is doing it's job, malabsorbing that fat and keeping it out of your arteries!
I get it all the time either from transfats! or chili. Looks the same going in as it does out. I can't believe this is the first time it happened to you. If I look at a transfat it ends up in the toilet. So weird how each of us malabsorb differently.