Negative BLOOD TEST, 5 WEEKS pregnant!!

I was over 2 weeks late and feeling symptoms of pregnancy, constant urining, tender swollen breast, fatigue, nausea, large appetite and hard swollen stomach. I took a blood test and it was negative. Took an hpt 3 days later and it was positive. Went back to the doctors the next day and blood test was negative. I demanded an ultra sound and found out I was 5 weeks pregnant with TWINS!