I missed two periods, i took three pregnancy test (which were negative), i had an ultrasound scan done and theres nothing there. im not experiencing any pregnancy symptoms apart from fatigue and ive been extremely anxious. what could be the issue?

Tests are accurate: 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.