The zygote is barely developing after day 2 (ie from full fertilization; presuming it takes 72 hours for an egg and a sperm to fully fertilize and materialize... based off the 72 hour pill, which can prevent fertilization).

Your uterus recognizes a pregnancy after about 15-21 days. Morning sickness and period absence are evident during that time, former just beginning then. So there's no way to know you're pregnant five days after having sex.

Not to sound like I'm repeating the others, but they are right. It's most likely the stomach flu. It is that time of the year, after all! Go to the doctor's if it doesn't get any better.