LATE... Update...

Ok so I thought I was out. 
Positive opk cd 14 and 15 
12/8 BFN CD28 (12-13DPO) then light spotting 
13/8 nothing
14/8 mildly sore boobs and very mild cramping
15/8 mildly sore boobs
I cannot face another BFN. 
Am I right in thinking CD 28 (12-13DPO) is too late for an implantation bleed? 
Update: 
AF seriously enjoys messing with me. Not 30 minutes after I begin to hope the cramps and spotting start. 
I'm done. I can't do this anymore.