BBT confusion!

This is my first month tracking my bbt.

I've been checking it every morning as soon as I wake up and it has been around about the same temperature...

However last night I was messing with my thermometer trying to work out if it shows the previous temp... anyway I took my temp and it was significantly lower than it was in the morning!

I thought thought ur temp was supposed to be lowest first thing in the morning? Do u think my thermometer is broken/ faulty?

*confused!*