Hm that's interesting! And it looks like it was negative again today. Same thing happened to me last cycle. It was negative the morning of CD 16 and I just felt different in the afternoon so I tested again later that day and got an almost positive, and a clear positive that night. If I only tested once on CD 16, I would have marked it as negative. The next day (CD 17) I had a small-ish temp spike and my OPK was still positive. I'm still not sure when I O'd last cycle as the time between the onset of the LH surge and O would only be a couple of hours if I go by that first temp spike on CD 17.
If you still have some OPK's left, I would keep testing for a day or two just to confirm that it wasn't a false alarm and a random temp spike (looking at last months' chart, you also had a temp spike before you O'd). Your CM also point to you O'ing tomorrow or so instead of yesterday as it should "dry out" pretty quickly after O.