BBT Chart?

Lauren • Adventuring through life with the one God made for me 💕
Is there any reason why my BBT is always so low in the morning? Should I be worried? Also, does anyone have insight as to why it's all over the place? I was supposed to start AF yesterday, 9/29. Told Glow I was late so it moved my period back two days, so I'm actually 15 DPO, not 13.