What is this big temp dip at 11 dpo?

Mrs. r
Ok I normally take my temp
At 6 am every morning , five or take a few mins . This morning I woke up at 4 to go to the bathroom and couldn't really get back to sleep. I dozed off for maybe an hr and just layed in bed until my time on my bbt went off . Why such a big temp dip? Afmaybe? 😔 I took a preg test this am it's a bfn.. Any input?