Hello, 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. However, nausea and diarrhea 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. Take care!