What Causes Delayed Periods?.

hlw sir i am 26yrs old nd my g.f is of 24, we had physical contact genrally but we use protection. last month we didnt use proction and we had within 24 hr of sex i gave her birth control pills nd again we have unprotected sex after 2 days and again i gave her birth control pill, now this month her period is not coming.plz sugest me what to do nd prescribe some medicine to bring the peroid back.. thanks