If its the day before or day you expect AF then it it is almost always a sign that AF is on her witchy way. If before that it doesn't often mean too much. Temp drops in general can be random temp fluctuations, mid tww estrogen surges (very common), "implantation dips", and other thing. Whether its below the coverline doesn't make a great deal of difference - the coverline does not have any physiological significance, but is more like a helpful tool to distinguish between pre- and post-ov temps.
Good luck!