What Causes A Delay In Periods After Intercourse?

hey doc , i had a unprotected sex wid my girlfriend on 1 june and i gave hr unwanted 72 and after 24 hrs she had a period for 2 days ,from 4th june to 6th june , and tilll now she has not got his period , even i gave hr pragnancy check kit on 2nd july and result was negative ,, can u just tell me at what date she will hv her period back