Crazy temps!

I'm on CD10 and so far my temps have been crazy! I take my temp at 6am each morning. For the past 5 months my temps for these first 14 days or so would hover around 36 degrees C (+/-). This month, they're hovering around 37 degrees C. During my period I think I was running a low grade fever as I was not feeling well (nausea and diarrhea) and my temp got up to 37.6. But now, I feel okay and they're still running at 37 (which is the same as 98.6 degrees F). The funny thing is, for the past few nights I've taken my temperature right before I go to bed and it's been back down around 36. What gives? I would think my temps right before bed would be higher than my BBT. My digital thermometer is 6 months old and I use it at least once per day. Is that their lifespan? Should I get a new one?