Pregnancy test question.

mamaX2

The past two times I've taken a test as the dye is filling window there is a white line where the positive test line would be but after the test is complete there's no line but if I hold it up to the light you can see it again. I took these two test like four days apart so i don't think it's a faint line I just don't know what it is. Has anyone ever had that happen?