BBT??????

Elissa
Ok so I just started taking my bbt last week. Well it was staying the same at 98.2 then yesterday it was 98.5 then today it dropped down to 97.8????? What could this mean? I'm due to start AF today but so far she's a no show. (Hope she doesn't show and I get a + later this week) Already took a pt the other day and got a BFN