BBT dip?

Lori
My husband and I have been ttc for 7 months. We started femara this month, and charting temps. This month I ovulated on cycle day 13 and my temp has been around 98.4 -daily. But this morning 9 DPO my temp dropped to 97.73. I'm having some lower back cramping. But AF isn't due until Jan 5. Is she coming early? Can any one explain temps to me?