L.A.
My sister had this exact thing happen to her, her belly even grew.
It is called a phantom pregnancy I believe and in my sisters case it was caused by an ovarian cyst. though you have had a lot of advice about it being a tubal pregnancy or even a miscarrage.
or, you very well could be pregnant too, wait a couple weeks and take another test! Hope everything works out
http://en.wikipedia.org/wiki/False_pregnancy