How much variation in BBT?

Hi everyone
​So I just started taking my BBT in the mornings...
​I am on my period so obviously not ovulating (!) but I have noticed there seems to be a fair bit of variation in my temperature. Up to 0.5 degrees C. Is this normal or is my thermometer broken or something?
​Obviously I need my thermometer to work to get accurate readings when I DO ovulate. 
​Any advice?