What Causes Delay In Periods While Not Being Pregnant?

Hi, i m 27 years, i have missed my periodsfor this month my last lmp was 10 jan 2011, i have done my urine test, it is negative, my gynac has told for HCG 0.0, prolactin 16, tsh test was normal, haemoglobin in normal range, knoe the gynac has given becosule caps for 15 days and asked for pelvic usg on 1 mar 2011 pl answer what is the exact problem.