BBT drop? confused...

Hi ladies! I've been charting my bbt and today saw something I haven't had before. I had a normal rise for 2 days after (I think) I ovulated (day after positive opk). Today, my temp went down my .17 degrees. I've never seen it go down after going up until the day before I get AF. I've tried to figure out what's going on by googling, but thought someone here might have an idea? Could it possibly be implantation drop?