Using the right way for your own BBT

So I'm worrying whether I'm getting inaccurate reading when taking my bbt, I take it the same time every morning and always have a good night sleep, all has good batteries, I use it orally but it always seems to be low, as in worrying low, I am not a month breather either, this morning I through I would do it the same way and then try vaginally after to see whether there's a difference and there was a massive difference. I am thinking of moving to vaginal tempting in my next cycle. Has this happened to any one else? Just some stories please. Thank you