Just started bbt charting help ....

I have been doing it at consistent times started later on in cycle ... Fri ... 36.3 sat 36.2 Sunday 36.1 ... today 36.8 no fever or illness period due in about 3 days .... is my thermometer crap? why the dip then sudden rise ? wish I'd of started earlier on my cycle ... any input ? should I order a new thermometer x