Emmy1, I'm sorry to say, but it may have been a chemical pregnancy. It's when the egg implants, but then falls away like a period often resulting in a positive pregnancy test and then a period a couple of days later. Many women don't even know they have had a chemical pregnancy, they just think their period is a few days late. You would only know if you are testing very early, as in your case.
Maybe you could try a few other brands? just to make sure.