BFN... No AF yet?

Lei
I'm on CD 36 (I think) and I'm 13DPO and my AF was supposed to be due today (I changed the period date to start tomorrow as AF hasn't arrived yet) and I took a First Response Early Response hpt and it came up with a BFN! What do you think I should do? Should I wait another couple of days to test again? I am getting some cramps and I think it's AF cramps but hard to tell. 
Has anyone else experienced this same thing and had a BFP in the end? 

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