My period usually lasts for 3 days, and a fourth day of 'might be over, might not'. this month my period started a day late, on the thursday, and finished on the sunday. My boyfriend and I had sex on the Monday, and he finished inside me. On tuesday I experienced some spotting, light pink and bright red. Then on wednesday and thursday continued spotting but it was more brown red. What does this mean? I have totally regular periods down to the minute, so this is very strange.

Could it be residual period? Or could I somehow have gotten pregnant in that one day, and already experience implantation bleeding (which would be strange i think)?