New at charting BBT...help!

Kim • ❤️🧡💛💚💙💜 Mom of 2 angels praying for our rainbow baby💜💙💚💛🧡❤️
So I'm trying charting my bbt along with opks for the first time this month. My average temp has been 97.6, but today on cd 12, it dropped to 97.0. Opks are all negative. I'm not supposed to start my fertile window until cd 15. Is it normal for bbt to drop that much?