Read the instructions. They might tell you what happens if you leave it too long, and whether one left all day might read positive by mistake. You can also go to the doctor's and get a blood test, which is more accurate. Or wait a week and test again, at which point pregnancy hormones, if present, should be much higher.
I would say that first; your period end? 2nd; are you eating a bit more than usual? 3rd; getting sick to your stomach usually comes around the first trimester...about the 3rd month. Those are clear markers if you are or not. If you should not get to this point or have any sharp pains, could be a tubal pregnancy and then I would go to your doctor regardless!
Eveningsun