Ng pg tests 8 days since period was due had black blood a week before period was due when wiped

Helli im 20 years old been having normal periods since my son was born almost 2 years ago but this month was weird my lasts period ended on nov 11 2011 haven't got it since my period was due on the 13 of December im 8 days late now i have took pee test all neg and i had dark bleeding like black when i wiped a week before my period was due on the 13th im kinda confused i have had unprotected sex in the last 2 months Need some advice plz help