Im 14 weeks pregnant and negative home pregnancy test?

I decided to take a pregnancy test i had laying around from when i found out i was pregnant, to my surprise it came back negative. How is this possible? i know im defiantly pregnant i just had my 12-14 week ultrasound 3 days ago and it showed a very active healthy baby measuring on size to the day. Maybe the test was just a dud? Any ideas on why i would now test negative?