BBT fluctuations

Hi,
This is my closes first month temping and I've noticed when I take my bbt as soon as I wake up, Because my head is generally covered with my comforter my BBT is high.. When I re check after a min without the comforter it drops down. Yesterday I got 98.3 and it dropped to 97.6 ... Which one is the right one?
How do you guys test?