What Could Cause A Delay In Periods By A Week?

Hi, I had a pelvic scan and the result goes this; 1. Non gravid, normal size anterverted uterus with homogeneous myometrical echo texture 2. Adnexae, pouch of Douglas are free That was the scan result My period has been 6 days late I did blood pregnancy test already and it also came out negative Pls what could possibly be wrong for me not to have seen my period