I'm not great with other BBT charts apart from those on Fertility Friend (I find the graphics quite confusing) but I also think it's entirely possible you ovulated Monday. I wonder if you input your data into FF and used the advanced - when FF thinks you ovulated.. The chances of conception with BD on O-day are actually minimal. The optimal time to BD is O-2 and O-1. I've copied this from my blog:
O-3 (8% chance of success), O-2 (28% chance of success), the day before ovulation (24% chance) the day of ovulation (8% chance of success) and on 1DPO (0% chance of success).
Baby dust!
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