What Causes Missed Periods After Taking Primolut-N?

Hi, My last period was on jan4th 2011. On 22nd ? I consulted doctor did urine test which is -ve and suggested me to use primolut-N. I used them from 5th to 7th feb for 3 days twice a day. Didn't get my period. Conducted self test but got -ve. Please let me know what may be the reason for this.