Hi..I'm 23 Years Old. I Haven't Get My Periods From

Hi..I'm 23 years old. I haven't get my periods from last 2 months. I gone through the urine pregnancy test which results NEGATIVE. I had also tested pregnancy 4 times by test kit after missed periods and the result was negative. I was prescribed by the Dr. for 'Norethisterone DLOT-N' tablets - twice in a day for 5 days. Please tell me do this tablet will suit me to get back my periods. Is this a right medicine to take in. After how many days will my periods starts? And what are other possibilities?
Please help me.