2-3days late, took a pregnancy test, with negative result, headach?

Pregnancy unlikely: Home urine pregnancy tests are very accurate by 3 weeks after sex (a positive test result is accurate by 1.5 weeks after sex, but a negative result may be a false negative if tested too soon after sex), or by 1 week after missing a regular period. So, a woman with a negative result at 3 days after missing a period is unlikely to be pregnant, but if still worried, she can test again 1 week later.