Charting BBT... Wrong!?!? pls help

I've been charting my BBT daily since 2/15, cycle day 8. It has been average between 97.1 & 97.8 except on 2/21 & 2/24, it elevated to 98.2 both days. I know they say your temp increases after ovulation & remains elevated until your period. I had a + O test thurs & fri. My temp was average those days & then slightly increased to 98.1 on 3/2 & then yest 3/3 was 98.6. I assumed this increase was due to ovulation. This morn 3/4 I temp'd & it's low again, 97.3.