Just a question.

Has anyone ever been 5 or more days late and still got bfns but still ended up pregnant? I normally have a 21 day cycle and I am at 13 dpo I tested yesterday and got a bfn so I am just confused. I see so many people getting their bfp after 8 or 9 dpo but I am wondering if the reason Im getting bfn is because my cycle is pretty short? When should I test again? Any advice is appreciated.!!!