Unprotected sex 5 days before missed period, bike riding 13 miles 2 days before missed period, 3 weeks late now, 4 neg hpt's (2 were taken today) why?

Some possibilities: (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… a visit to the ob-gyn doctor is a good idea if the next period is also missed, or if worrying.