Anyone know much about BBT?

Angelie • Married to the love of my life! Puppy-mom to a Dorkie pup named Lilo. Now praying to be blessed with a baby!
So AF is due on the 19th. 
My normal BBT is in the 97.3-97.5 range. Sometimes 97.7. 
Checked it this morning and it was 98.5! I've never had it that high in the 3 months that I've been tracking it! The highest it's ever gotten is 97.9 
Any idea what that means?