Yo should be below a 97.6 I think in order to be low. You have to see a degree jump not a little fluxuation. Be patient and good luck.
A temperature drop can mean anything: AF is coming or ovulation dip or today in TX we had a cold snap (i.e. 60 degrees :)
I think you can only interpret BBT looking backwards but not forward. Temps going up or down can mean anything. Don't read too much into this. Besides, a temperature change is considered significant if it's over 0.2. So, you probably just observe random variations.
You can't worry about temps dropping a tenth of a degree or two. It's all about the whole "range" of temps. My temps will go up and down from about 98.1 to 98.4 the whole LP and you can't worry, as long as they stay within that range of temps.
Good luck!