Im not sure...From what Ive learned it is a possibility that could be a dip due to implamentation. Now just watch your chart to make sure you dont go below the cover line...Good Luck!
A dip in your chart can be good sign but sadly it can also mean nothing. If I remember right a dip below the coverline is a more possibility of an implantation dip! A dip around the time AF is due is likely to be AF so don't confuse the both