BBT drop after waking up

I'm confused on how bbt works. I've been taking my temperature first thing in the morning before getting out of bed or even sitting up. My temps have been all over the place, so our of curiosity I took a second temp after I had breakfast and my coffee before I got dressed. My original bbt at 7am was 97.87. at 8:30am it was 97.38. I thought your temp was supposed to go up after moving around and that's why you do it before moving. Does anyone know why that might be? Because that happened I just tried my temp again and it was 97.68 at 10:30am. I'm so confused.....