Diff bbt readings taken consecutively-same time/same day

I’m so confused. I’ve been taking my bbt at the same exact time first thing in the am. But when I temp more than once, I get vastly diff readings. This mornings first reading (which registered in like 30 sec) was 97.31, second was 97.82 and third was 97.69. The second and third readings took much longer to register. This isn’t the only morning this has happened. I’ve been using the avg temp between the 3 (not easy to calculate at 6:30 when my brain is not fully awake). I’m temping one right after another. All within the same 3 minutes, no moving, etc.

Anyone else experience this? Starting to think I need to upgrade from the cheap (but new) generic cvs thermometer.

This first temp just always feels off to me, especially when it registers like immediately after sticking it in my mouth. Def under the 60 sec it’s supposed to take.