I have missed my period, feel dizzy, headaches, diarrhea, and nauseous. However I took a pregnancy test and it was negative. What could be wrong?

Vague symptoms. Menstrual irregularities like a missed period can occur with an infection or a non-infectious inflammatory process or even psychological trauma. Anything that alters hormonal balances can do this. You might want to use another home pregnancy test on a first am voided urine specimen in a week and make sure, otherwise wait and see what happens next month.
Home pregnancy tests are very accurate. They are the same tests used in a doctors office. They can detect a pregnancy 7-10 days after conception. The best time to take one is when you miss your period.  if you doubt the result you can repeat it in 1-2 days. If you were pregnant the hormone level would rise enough for detection.

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