Unfortunately, I think this is one of those "every woman is different" things. It's funny you ask this, though, because I was just re-reading TCOYF last night and it was saying that some women can sleepwalk through snow and not have it effect their waking BBT - others are far more sensitive.
Is your bedroom warmer than usual? We generlly don't use AC (unless it's well over 90 degrees outside), so sometimes on warm evenings the bedroom gets a little hot. For some reason, though, for me personally this doesn't seem to make a huge impact on my temps. I've sweated through a hot night, and then gotten a 97.1 reading in the morning.

Did you have any other signs of O - such as CM, high cervix, OPKs?