Miscarriage question *trigger warning*

So I was late on my period. Got multiple positives but lines were progressing so I assumed the worst already. I ended up passing clots all day at 4 days after missed aunt flow. That lasted one day and that's it.

Well I was having weird symptoms and literally felt like shit/dizzy so I went to ER because I felt like passing out.

He drew blood and it was "negative" for pregnancy so he diagnosed me w miscarriage. My question is- could blood be negative even if the dead egg is still in there? He didnt do an ultrasound so how does he know it passed out of my all the way?