Thanks for your query.
Periods can be delayed by any stress (physical, emotional,infections etc) and pregnancy
test may come false negative in first week.
may constitute gastroenteritis
. It can be caused by variety of bacterias, parasites and toxins. You need to check your hydration and take plenty of water and other fluids. If it dose not resolve within a week then visit your doctor and analyze a stool sample. Meanwhile perform blood HCG level to rule out pregnancy.
Hope it helps.