Bbt after period

I've only been charting for a couple months and everything has been "normal" until this week. On Monday my temp dropped back down below my cover line and stayed there for 3 days. Had a very light period but that's pretty normal. Only one time that I "needed" to change my tampon (tmi). Now for the last 3 days my temp is back above my cover line and slowly getting higher. What in the world could that possibly mean?