so confused

I read that implantation bleeding can show as pink or brown spotting...my "period" was 2 days late and then I just had 2 1/2 days of brown spottjng...no cramps no sore boobs as with my usual AF...I'm having lower back cramps and mild headaches daily along with bouts of nausea (not too bad)! My spotting lasted from the 17-20...I tested on the 24th but was negative...should I test again or hold off and count the spotting as my AF?! Any thoughts would be appreciated!! UPDATE: if I was miraculously pregnant...shouldn't it be showing up by now?