Could be an evap, indent, you could have tested using more diluted urine after, if you used a different test it may have been less sensitive, if you tested at a different time of day hCG concentration may have been less, could be a faulty test... lots of things, only thing you can do is wait for af or test later on after a few days and see what happens but I would suggest using a couple of different brands, in case it was a faulty batch