BFN... No AF yet?

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.