What Causes A Delayed And Shorter Period After Unprotected Intercourse?

Confused period 2 days late, had unprotected sex about 4 weeks ago tested 4 weeks ago it came out neg and then a week after that came back also neg, I haven't had sex since the 10th of November and had a strange "period" for 3 days (period usually lasts 7 days), Did my DR tested too early?