What are my chances?

Hi ladies, today 12dpo my temps taken a big dip. I'm confused as it's at the same rate of decline I usually see for AF coming but it's a day or two sooner than normal. I feel deflated as since weds iv been sure I was pg (despite no BFP yet) as iv been feeling nauseous, especially wen talking about food, aching back, had a sharp stinging/stabbing pain on the right weds which iv had twinges since and felt convinced was implantation pains. A posterior cervix and been tired.