M.L.
I had an experience very similar to what you are describing. When I got the the DR's office, she said my test was Negative ( after 3 positives at home)and she described what is called a chemical pregnancy. Essentially, the fertilization occurs, but it doesn't implant correctly, so your body thinks it is pregnant for a few days.
The VERY NEXT cycle, I became pregnant with my adorable son. I wish you luck- don't stress out about it!