Help reading bbt chart?

Do you think ovulation was pinned accurately on CD18? And what is with the dip yesterday? Could it possibly be an implantation dip? Have been cramping mildly last 3 days. Don’t want to get hopes up, but temperature doesn’t usually drop until cycle day 28, just before AF arrives. I’m aware it’s early, but any thoughts? New to charting, and input is appreciated.