What Causes Fluctuations In Temperature During Menstruation?

I am 26 years old. Three days ago I checked my temperature and it was 99.1 my normal temp is approx 96.7. The next day I started my menstration cycle and I checked my temp again and it had went up to 99.2. Today I checked my temp again and it is now showing 99.3. I have never noticed a temp before duing my cycle. What could this mean?