BBT help

Monike • Me and my fiance have been together for 4 years and are ttc #1
Hi wonder if you lovely ladies can help me, bascially after ovulation my bbt has been rising slowly but has mainly been between 36.56 and 36.57 but at 12 DPO my BBT dropped dramatically to 36.39, so I was certain I was out this month and AF was going to appear any minute but then this morning 13 DPO my BBT spiked massively to 36.60 , could you tell what this maybe?