I had unprotected sex when my period was due and didn't get my period, it's 3 weeks later and due for my next period and a negative home pregnancy test. what should i do?

If no next period...: 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. Home urine pregnancy tests are very accurate by 3 weeks after sex.