Positive dollar store test and neg clear blue digital

Brittany • Expecting our second April 2015 ?
So I'm expecting AF tomorrow , and I've been <a href="https://play.google.com/store/apps/details?id=com.glow.android">tracking ovulation</a> . I've gotten 3 positive opks in the past three days . So I took a pregnancy test it was a very faint line that kinda made me question if I was making things up in my head (my husband said i was crazy ) but I know there was a line . So I went and bought another same thing !! But then I wanted to know for sure so I bought a clear blue digi and it read "not pregnant " what would cause this ???