I can understand your concern.
Based on your query I would say take it easy on yourself with the sex.
5 days after her periods end considering she had a 4 day bleeding and her periods are pretty regular, she would be in her semi fertile phase of her cycle, there is 30-40percent chances of getting pregnant in this phase roughly.
And considering that you have taken the pill within 7 hours, the chances decline further.
So I would say a chance of about 5-10 percent.
Next time don't have unprotected sex and also, you are only 18. You don't want to be getting pregnant at that age.
Dr. Anil Kumar Jain.