I was about to have my periods but i had unprotected sex one day before my periods. ive missed my periods for 16 days & test results were -ve . ?

Can repeat preg test: Pregnancy is the most common cause of a missed period. If not pregnant, other causes of a late or missed period include: excessive weight loss or weight gain, increased exercising, emotional stress, having an illness, traveling, medicines (especially birth control pills), hormonal disorders, etc… One can repeat a urine pregnancy test one time. If worried, one's primary care doctor can help.