Taking combined birth control pill regularly on scheduled time gives protection near about 99.9% against unwanted pregnancy
Unprotected sex before period due date is quite safe and you also got your period there after. So you have nothing to worry regarding pregnancy.
Period date may alter up to 7 days and it occurs due to hormonal
imbalance & mental stress
Take healthy diet with iron rich food stuff, avoid stress/ anxiety
, keep bowel habit regular, take sound sleep.
Practice safe sex in future. Good luck.