Why is working out ovulation such a nightmare

So my period finished last Friday. We BD that night.
I've had some moderate CM over the last couple of days and now today zip nil nothing! Dry as a desert! 
I did opk Sunday- Wednesday and I have had no surges.
Is it possible I've ovulated during or straight after my period finished. 
I've been off the pill since June so my cycle is still adjusting a big. In my teen days I had a 21 day cycle. Last month was 24 days.

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