What Causes Delayed Periods With Negative HPT?

Hi i am 26 yrs old married woman and was having irregular period problem so doctor advised me to take Krimson 35 tablets for 6 months. I have taken that for 6 months, the course got completed last month in Feb. And my last period date was 18 Feb. but today is 21st march and i am not having my period yet :( I am so worried about it. I tried to concieve in between Feb and March and i took preganacy test too but i am not preganant :( Can u pls tell me whats the problem is? And i want to get preganat so what should i do?