Your current dark brown vaginal bleeding (similar to period) is due to withdrawal effect of the unwanted 72. It is quite common phenomena following taking the emergency pill and bleeding starts within 7 days. You don't be worried/tensed as it stops automatically.
You were involved in unprotected sex around the safe period zone as per calculation of ovulation time (as it occurs 14 days before next due date). Also you took unwanted 72 on next day. So there is no chance of pregnancy
as it is more than 99% effective in preventing unwanted pregnancy.
Your next period may be delayed or earlier due to hormonal
imbalance associated with unwanted 72. Practice safe sex