thermometer readings fluctuating

My oral BBT readings have been all over the map for the past number of months with no pattern. The thermometer that I have beeps after 5 minutes, and I've noticed that if I leave it in for up to about 5 minutes beyond this, the temperature reading continues to rise (sometimes by up to 0.5 deg C). I recently purchased another BBT (BIOS digital ovulation thermometer). With this one, I am also noticing that if I leave the thermometer in my mouth for a few minutes beyond the time when it beeps, my temp reading continues to rise. I am unsure whether to trust the initial reading or the reading after I have waited for minutes beyond this. Insights welcomed!

Thank you,

Jessie