Period pushed back

Katie • 26 y.o. TTC #1 since Jan `14. Took out last nuva ring Nov. `13.
I'm on cd42. My last cycle was 42 days and glow was predicting af to show up tomorrow(2/3), but today it's been moved to the 11th. I do think I may be pregnant and I'm pretty sure I'm 16 dpo, but have only gotten bfn. What is glow using to determine my period? The only thing I can think of is bbt which has been consistently elevated since last week.