I'm finally filling out all my FF data for the last year or so, sotohana, and gave yours a peek since I totally have charts on the brain!
Now, I'm no expert like some of the people around here, but I'm thinking you O'ed on CD15... Based on your CM and OPK. Yes, your temp dropped on CD16, but then on CD17 it went right up higher than it had been on CD15... This could be the start of the typical spike that comes after O'ing. What does your temp typically do after you O, if you have that data? Mine is more of a steady climb than a spike, so I had to learn to recognize it.