What Causes Delayed Periods After Having Unprotected Sex And Taking Birth Control?

hi.
i had unprotected sex on 1st and have taken an i-pill on 3rd..my periods was to start on 5th. i have also taken mensovit plus but still i have not got my periods.i have stomach ache..
pls help me in this regard. i m really worried..can pregnency take place in safe peroids?? i had regular periods til last month..
pls help