Don't panic, Spuddles! Do another Discover One Step early pregnancy test today, they're more sensitive. It could be that your FMU was a little watered down. Did you look at the blue lines on the digital test? If there's a definite second line there, you just mightn't have had quite enough hCG to kick it over to say "pregnant". But by tomorrow you should have enough for that magic word to show up on those expensive hpts - it's soooo good to see it, isn't it. No, I don't think it's possible to get a false positive on the digital tests - just false negatives!
Congratulations on your BFPs. Book an appointment with your GP and I wish you a very happy and healthy pregnancy.