Whats with the pre-AF temp rise AGAIN?

I'm 11dpo with AF due tomorrow. Today I had brown spotting after sex and a temp rise. I'd think this was a good sign but last cycle I had the EXACT SAME temp spike with spotting on 12dpo. 
It was a weird cycle, with AF a day late and suspected chem pregnancy (BFP 8dpo, all BFNs after). 
Why would I keep getting a temp spike the day before AF if not pregnant? 
Just need some honesty really, can't keep convincing myself I'm pregnant only to be thoroughly UNpregnant. Had nothing but BFNs on tests so far. Thx

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