how late can your period be if you're not preggo?

I'm 4 days late and just got a BFN! :( recently had a miscarriage in October too. We've had no luck since then. I got pregnant with my first child after 3 cycles, the second time took 4 cycles (I was still breastfeeding the first three cycles) then we lost it :( now it's been 4 cycles and nothing. I've never been this late even with my irregular cycles. I swear I'm pregnant but I did a clear blue digital and those are supposed to be 99% accurate :(