BBT drop 13 DPO but still above coverline, not pregnant?

My BBT chart looks good..but my temp dropped overnight from 98.4 to 97.8 (after a night of night sweats and interrupted sleep). My coverline is 97.45. I have no other signs of impending period (I usually spot a day or two before, have cramps, I feel nothing). Any idea as to what is happening?