Hi My Friend Had Unprotected Sex On 15th Jan And

Hi my friend had unprotected sex on 15th Jan and her last period started on 31st December and ended on 5th-6th Jan, Immediately after sex she took Ipill within an hour and she had some vaginal bleeding on 23rd Jan, she thought her period has started but bleeding did not continue in the next day i.e. 24th. Can you let me know why this bleeding would have occured is there any possibility of pregnancy?