Unusual spotting......UPDATED!!

Hi ladies....AF is due to arrive tomorrow, I have a steady 28 day cycle and ovulated around day 15 this month. For the last week I have felt crampy but just put it down to a sign that AF would be arriving. I've taken tests and all are BFN. Today I woke up just knowing that AF would show.....when I wiped there was a small amount of blood, just spotting. I assumed, 'ok, here is AF!' But since then, nothing. Normally my first day is very heavy so I'm confused. When I wipe there is literally nothing at all but I am still achy and crampy. I'm wondering if it's too late for implantation bleeding?