What dipping

Melissa • Married 9/16/2017 💍 - TTC since July 2017 🤰- two step sons 🙋🏽‍♂️🙋🏽‍♂️ and two fur daughters 🐶🐶

So I get my AF tomorrow and my BBT dip today and I was just wondering if anyone can help out any explain is dropping like this normal?