False positive?

Took a pregnancy test today. Soon as the lines come up I seen straight away that it was an obvious negative Not sure how long you're supposed to actually wait to see your result but I assume straight away? Anyway left the test on the bath and walked away from it. Came back about half hour ish later and it was showing a test line too so a positive. Assuming it's just a false positive right and probably the evap line or something because of the time