Help me understand It? Bbt

I know, should have waited till I started a new cycle to start bbt, but I started near the end, just I'm confused because about 8 days ago I used an opks and it came back as peak ( digital smiley flashing, plus a normal strip test that glow said was peak), just thought there be a longer period of heightened temp, it did go back down but today its went back up 97.28f even tho I started my period