BFN at 12dpo is this cycle a bust again!!?

My cycles have been a little all over the place but last cycle was 40 days with 12 days between ovulation and period. My BBT chart showes that i am 12dpo so if its the same as last month im due for AF today. I got a little excited this month as I had a bit of brown spotting on 10dpo so took a test this morning and BFN.. If my spotting at 10dpo was implantation should i be showing BFP by now if i am pregnant?