IB or Old blood?

Okay, so... 12 DPO I started having a discharge that was a dark brown, it's been very spotty and light and has been happening for about 3 days, so far. I'm supposed to start AF in 3 days, as well. It started out looking like old blood and now is starting to fade and go away. 
Could it be IB or is it more likely just old blood from my last period?
If it is possibly IB, when should I test for best results?
**also, I took Plan B before my last period and it really messed with my cycle, could this have an affect on what I'm experiencing now?