Basal body temperature confusion?

I am trying to pinpoint when my o is using BBT, CM and OPK. My BBT drops yesterday morning. My opk was -ve whole of yesterday (last reading at 8pm). This morning, my temp went back to normal but not really spike. At 10am this morning, my opk was +ve. Does it mean o has occured but why temp did not go higher than normal ? Moreover, I thought it will occur 12-24 hours post opk. Any opinions?