I read your post today and researched on line and came up with the same things you did. They based it on environment and summer/winter, etc.

I also run a low temp. Fluctuates in the 97 area. If it runs 99 or over, that is a sign of something going on with me. Like curious asked, was your norm low before and just got lower?

Mine was always close to 98.6 until I was in my early 40s and then it changed. I think too that some meds can change body temp.
Sometimes I think the new thermometers are junk.

Give me the old mercury ones any day.

Hope you can find the answers you are seeking,

Judy