If you are sexually active and missed your period by 8 days, and you took 3 pregnancy tests that said negative, why wouldn't you get your period?

Contraception used?: If contraception was used, then your period is simply delayed. If not, see your OBGYN MD to rule out pregnancy.

Irregular periods: Sometimes you can skip a period or develop irregular periods. It's not uncommon, and it's not usually cause for concern.

Other possibilities: In a woman with regular periods, some causes of a late period include: excessive weight loss or weight gain, increased exercising, emotional stress, having an illness, traveling, medicines (like birth control pills), hormonal imbalances, etc… Seeing a doctor is a good idea if symptoms are persistent, recurring, or worrisome. Pregnancy is the common cause of a missed period, if sexually active.