Confused test results

Ma
I took a no name pregnancy test early in the morning.. Checked back maybe 5 minutes later and it was positive... I wanted to be sure it wasn't a false positive so after a bit of running around this afternoon I took another no name test.. And this time the result was negative! :'(
I've since went out and got a couple others to try with tomorrow morning... But my initial joy has since been replaced with doubt and sorrow....  
Which test should I be more apt to believe???