My mom couldn't get a positive anything with me until 5 months. If you have any doubts about what your provider has said or done, you need to find a health care person who can help you. I am sorry for your loss.
First of all, I'm very sorry for your loss. Are you talking about a blood serum test (as opposed to an at home test?) If you've had a quantitative beta hcg blood test come back negative, in all likelihood you are not pregnant, though I suppose anything is possible... if you believe the test is wrong, you can ask for an u/s to be absolutely sure what is going on.