Im sure you have no need to worry. Some of the things that can cause higher temps are: medicines, stress, excitement, the weather, different sleeping habits, sickness/allergy that isnt bad enough to notice or, if it stays elevated and continues to worry you... get a blood test to check hormone levels, but really with only 6 days of still-high temps I would chalk it up to one of those other reasons.
and the 18 day rule only counts if you dont have AF during that time, which you seem to be saying you have.