What Causes Irregularities In Menstruation?

my girlfriend was due to have periods on 10th jule while we had sex on 2nd .... though we used condoms but still she missed her periods ... her past history of havng periods is very accurate infact she has thm a day or 2 in advance ! on someone' advise she took the 1st dose of gynaecosid on 12th july and the 2 nd on 13th ..... but she didnt get her periods as yet .... wht to do now as we are really worried n v dnt want this pregnancy ???