What will be the other reason your period will delay after having sex except pregnancy?

Some 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… A home urine pregnancy test, done correctly, gives a reliable result 3 weeks after sex. Pregnancy is the common cause of a missed period, if sexually active.