BBT irregularities?

MissRdotS

Hello all, 

So I went and bought a Mabis BBT from Walmart earlier this week and feel like maybe I'm not doing it right? I wake up at 7am everyday, so that is when I test. I put the tip of the thermometer in the "hot spots" as shown on the diagram I got with the test and waited until it beeped. Yesterday I got 98.58, and out of curiosities sake, I tested again immediately after and got 98.67 and right after that 98.42. This morning when I woke up I got 98.63 and then 97.73. I feel like this means I'm doing something wrong - any insight or tips?