Low BBT post ovulation?

Hi - so I’ve been doing my first cycle monitoring this cycle and went last for my blood work and ultrasound on CD 14 where the doc told me that I was going to ovulate so I should have sex for the next three days and come back on CD 17 to confirm ovulation - which they said it did. My concern is that my bbt look really low...? They confirmed ovulation via ultrasound on CD so I know it happened.

Doc said I ovulated on CD 16 (or else he meant Oct. 16? Not sure I should have clarified)

Anyway, my question is for anyone who can explain why my bbt are so low post ovulation? 😬