What Causes Missing Periods?

hi i am not having my periods for last 20 days,i have test pregnency test on 10th day it was negative,i went to my dr.she told to wait for nother 5 to 6 days,again after 6 days i test it was negative.dr told to take primolut nor 10mg for 5 days i have taken still i didnt get my periods,i am 27 years old and i have 8 months baby