Bbt questions.

My bbt has obviously stayed high over the second half of my cycle, but it dipped really low 97.2 (after ovulation) during my 2ww. It jumped back up into the 97.9ish mark, only dipped for one day. I couldn't have ovulated, I had a confirmed opk on the 22nd and 23rd of this month. Any opinions on that?? I'm only 8dpo now so I took a test knowing it was too soon and I can't tell if there is any start of a second line there or not. But why would my bbt do that ? Never has before ..