it's very strange, have your cycles been irregular at any point in the past? how far has your temp dipped? How does your chart compare with previous months? do you have a link to the FF charts to compare? (sorry lots of questions!)
It's very frustrating, until you get either AF or a clear BFP it's really hard to say at this point. It could be a number of things, you may have ovulated later than expected, perhaps that dip at 12dpo could be implantation? if so it would probably not show on a test until about Friday as it sometimes takes a little longer for a +ive test to show, or perhaps it could be something to do with the thyroid issue?
Nausea can be caused by the hormones, I've been getting tension headaches which are affected by them too.
I really wish I had an answer right now, it's like you're stuck in limbo and until AF shows up, its really hard to say