Who's experienced the hook effect in more than one pregnancy?

My pregnancies never show up on urine tests. My first son, didn't even know I was pregnant with him till I was 12 weeks along.

Second son, I had a blood pregnancy test at 5 weeks, which came back negative. Two weeks later I went to the hospital, they tested and I got an ultrasound, turns out I was 7 weeks 4 days.

So I've been feeling extremely nauseous. :( I've had a pregnancy test already which came back negative. I cant shake this feeling that I am pregnant.