Food poisoning is my first thought, and probably the easiest explanation. Even with making a concerted effort to avoid problematic food, I still get it at least once a year. Sometimes it's diarrhea and vomiting, sometimes it's just diarrhea, but it always, always makes me feel like I'm practically dying. I've been super sick in the past, out of school for a week-type stuff, I've had dental surgery, and I still can't imagine anything worse than food poisoning. It's the fucking worst. So yeah, that's what I'd guess, especially if it seemed like it came on relatively quickly.

Hope you feel better soon. ♥