BBT confusion. help?

Okay. so i've been charting my BBT regularly. And i check it right when i wake up, before phsyical activity. it's low. below 97.4, consistently. but i check my temp at the end of the night and sometimes its like..99. I know that if you have a high bbt that means you could be pregnant. what if i have a slight fever in the evenings? is that still an accurate measure to tell if i'm pregnant or is the only accurate one the one i do in the mornings?