How late is too late to test positive?

Pennie • BFP on April 6th... US complete, HB of 176. EDD of 12/14/18. 2nd US on 7/27 but waiting till birth for gender!
I have heard lots of stories of women being sure late for AF and still testing negative but then eventually getting a BFP. I am currently CD37 14dpo 4days late
Had implantation bleeding at 6dpo
Used opk, bbt, preseed, prenatal vitamins this month...
All tests are negative, including the Firat Reaponse that I took this morning. I have been having mild cramping for 6 days, some motion sickness, indigestion, nose bleeds, and more recently my breasts feel...swollen? I think that is the best way to describe. I am driving myself crazy with googling everything under the sun but what I really want to know is your stories and how they turned out. I also want to know at what point you should see your doctor if AF doesn't show. 
Thank you in advance for taking the time to respond. 

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