Confused about BBT

This is my first month TTC and I am also new to temping. I slept in until 8:30 this weekend and took my temp as soon as I woke up. I got up for work at about 5:30 this morning and my temp was 97, which was -1.3 from yesterday. I took it again a minute later, and it was 96.6. I waited 5 minutes, and it was 97.9. I only have a forehead scanning thermometer, so I am wondering if that could affect it as well. Does such a significant drop at 5DPO mean I should expect AF or should I consider today's temps invalid?