BBT chart help please

Temp dropped below cover line yesterday and AF due today. I've been spotting for the past few days and having slight pain on my side (not the usual cramps I would get for AF) but I do feel like it's coming because of the spotting (I usually experience spotting days before AF comes) I'm now 14 dpo and got a BFN so I know I'm out this month. But what I want to know is why did I have a rise in my temperature again this morning? Shouldn't it stay below the cover line? Anyone else experienced this or might know?