Negative blood tests

Has anyone ever experienced a negative blood tests and still be pregnant. I haven had a period since December 10th and I've had unprotected sex, I've gotten a negative blood test but I'm just not feeling myself. My body is changing and my belly looks different and feels different, I've had many symptoms of pregnancy. My doctor ordered a ultra sound to check it out because he was kinda concerned. I think that they think the blood work was false. I've heard of people not being able to get a positive because of there hormones being out of Wack, they said it could be a hemorrhaging cyst but when I called yesterday she was surprised that I didn't bleed yet. It seemed like she was sure I'm pregnant. I honestly have been just waiting to see but I feel in my gut that I am and so doesn't my boyfriend.