I am supposed to start my period tomorrow

I had bad cramps last night but no period. I took an at home pregnancy test and the first one I did was immediately two lines very clearly. I just took a second one and the line is so faint I can see it when I put my phone flashlight on it. I've heard if the second line is faint it is still a positive reading. Is there any reason why the first one would show up so dark and the second so faint?