fisher I'm so sorry you have this worry, but just remember a lot of "positives" from these blood tests end up being false. The fact that your Nt measurement was fine, plus they didn't notice anything untoward at your 12 week scan, means it could well be a fluke of bloods.
I think the 'high' hormone they're talking about is HCG (read up a lot about the Down's screening before hand) which needs to be of a certain height to sustain the pregnancy.
Out of interest did you have very severe morning sickness? That often happens with a higher level of HCG, and some women just have higher levels than others.
You must be going out of your mind but try and remember that most don't end up having babies with Down's, look at frozen! Her odds were higher than yours, and her tests have come back all clear 