HCG levels

Shelby • 25 yrs old... Mom to two beautiful baby boys & 3 angels. praying for H&H third earth baby
Ok so most of you know that I missed my period in March and kept getting negative results even on my blood test April first.
Well I finally got a positive on the 19th thinking we conceived in April, and dr office confirmation call came today and told me that my hcg levels put me at about 7-8 weeks which coincides with my lmp... So why  wouldn't my April 1st blood test have been positive? Or did I conceive in April and its twins??? So many questions and theory's running through my head... Anyone have any advice? Or experience? Or knowledge?