What Causes Delayed Periods Despite Taking Meprate?

My period delayed for 4 day. I did pregancy test negative then I went to doctor she adviced for meprate 10mg for 5 days twice a day . In that 5 days last two days we had unprotected sex . After stoping 5 day course . 3 day after also i am not getting periods . I tested pregancy using kit its negative ... kindly let me know still I have chance of pregancy ??