This could be an implantation bleed, so don't give up on the pregnancy theory just yet.
Although implantation bleeding is usually quite light, it can certainly be heavier and can last for anything from a few hours to a couple of days so keep those fingers firmly crossed!
If your period normally lasts longer, this again is a good indication. Was the bleeding darker than your normal period? Implantation bleeding is normally darker as it has further to travel and is, therefore, older blood. Quite often, cramping is felt at this time, have you had any cramping feelings at all? This happens as the uterus changes and the egg implants.
It's certainly worth doing another test now, or in a weeks time, and hopefully the line will be clearer this time.
My thinking is that you were not 'pregnant enough' when you took the tests for it to show up too well, but now may be different.
Good luck, I really hope you get the result you want.