17dpo, BFN, temp went back up

This is so annoying. Im on 17dpo, had a temp drop at 14&15dpo (not below coverline but still temp drop) and a bfn frer on14dpo. Thought AF is going to show up 16dpo so didnt temp, but still no AF and late evening took a dollar store test since its all i have left and stark white, CP high and soft.

This morning at 17dpo i took my temp, curious, and it went back up. It's just so annoying and disheartening :( why the mixed signs if BFN?

What could cause my temps to go back up at 17dpo?