My period is late by 18 days. ive never had a late period. took over 10 hpt tests all neg, as well an ultrasound & blood test. what could it be?

Possible reasons...: 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 tests are very accurate by 3 weeks after sex.