I'm sorry to hear that Claire, it must be disstressing for you.
I'm no expert but what may have happened is that you were pregnant, enough to produce some of the hormones that can be detected by the very sensitive tests now available, but if the egg does not implant properly, then the pregnancy does not progress, the hormone levels drop, and you are no longer pregnant. It is almost like having a very, very early miscarriage.
I believe this is extremely common and most women may not have even known that they were pregnant.
x