There are many things which can affect our BG.
Most people know about the carbs in what we eat (and for some, protein, too) but it can also be affected by time of day, month, year, illness, stress, exercise, medication, alcohol, ...
So, you niggling cough may be raising your BG - I sometimes find my raise BG is out of proportion with a niggling cough/sniff/... and have decided this is because my body is busy fighting off bugs which have not caused much problems yet.
Or it may be the stress of the up coming blood test (or something else)
Or it may be something you ate yesterday
Or it may be ....