So confused!!

Jaimie • Married to my High School Sweetheart 05/10/13. Miscarried our angel baby, Skylar Grey on 04/19/14 .. Adopted our little boy on 11/17/2017 & just had our last little boy on 08/30/2018👼🏼👦🏼👶🏼
So I got a POSITIVE pregnancy test last Friday 
And I went to the doctor to get blood drawn to check my HCG and she told me it came back negative?!?!? Whhhaaatt?? How is that even possible?  I'm 4+3weeks today..... 
Has this happened to anyone else? This is also my second pregnancy and the first one wasn't detected until 6.5 weeks (with home test or blood).