Why Am I Having Dark Brown And Black Coloured Discharge During Periods?

i am 27 days old married women. i have regular mensural cycle. i had my last periods on 29th dec'2010 for around 3-4 days. After that from 12th jan till date I ham facing dark brown & black colour discharge not like normal periods almost 16 days are over pls help wats going on in my body? is this sighn of pregnancy. i took hpt on 16th jan but it came negative.