Does anyone know what this might mean

Hi guys! So I’ve only just started tracking my bbt from 6dpo as I was waiting for it to be delivered. I tracked it from there and it rised, now 9pm it’s had a sudden drop? My period is due in 5 days time is this because of that or would that be an implantation drop, I’ll test again tomorrow morning at the same time and see if it goes back up. It’s been years since I’ve used one of these! (Ignore day 25 that was me logging in the evening so that’s an inaccurate result)