First time charting BBT - implantation vs AF ?

Je

Just looking for experiences with BBT dips post ovulation but before AF is due. I'm very regular (every 28 days) and today's temp was a lot lower than the last few days post ovulation but AF is not due for 5 days so I'm confused what this could mean if it doesn't go back up tomorrow. Also, does an implantation dip below the cover line ? And another question if it's an indicator of an abnormality? Any insight or experience is appreciated .