Suggest Treatment For Absence Of Periods

I am concerned!! I had a period on Sept 4, 2010, and didn't have another until January 6, 2011...it is now March 18, 2011 and i still haven't had one...before i started not having them, they were regular, normal, monthly cycles, i have never been on birth control...i know i am not pregnant, took 5 at home pregnancy tests and were all negative, and in Nov 2010, i went to doctor, and they did urine test, and was negative, and also did blood test and came back negative, and normal, and now i am concerned..why am i not getting a period, mt husband and i have been trying for about 2 years to have our second child...but it's kinda hard to get pregnant without having a period...please help, very concerned....