BBT Drop at 2 DPO?

Hi all - this is my first month charting, so I'm not really sure what I'm looking for. All signs (positive opk's, cm, and other symptoms) point towards me O'ing on CD 15. My bbt also spiked the day after that. BUT... Today my bbt is back down super low. Is anyone familiar with this pattern? I can't seem to find similar charts anywhere. Thanks for your help!