This may sound stupid but when I first started taking my temperatures I would read the thermometer when it beeped and it read really low, then I started keeping it in really long and trying different areas under my tongue and can get higher readings depending on where I hold the thermometer and for how long- I also get different readings on different sides of my mouth. so maybe your temps are higher than you think?
It would seem strange that you only have one sign of low metabolism, but I am no expert.
About thermometers – I’ve tried different ones and i get similar results among the different types.