I temp in celcius too. That sort of drop would be enough for me to consider that AF was only a few days away. Do you have a link to your chart?
Funny sleeps can definitely play havock with your temps. How many DPO are you? Have you tested?
When you shed your lining and get AF, your progesterone drops. It needs to be low so you have your period and start a new cycle. But if your progesterone levels are low after you ovulate, that's when problems arise.
Just from what you've said, if you've tested and it was a BFN, it's likely AF will arrive in the next day or so.